Fast PHP templating engine

0.8.0 2017-03-09 05:41 UTC



Flow began life as a major fork of the original Twig templating engine by Armin Ronacher, which he made for Chyrp, a now-defunct blogging engine. Flow features template inheritance, includes, macros, custom helpers, whitespace control and many little features that make writing templates enjoyable. Flow tries to give a consistent and coherent experience in writing clean templates. Flow compiles each template into its own PHP class; used in conjunction with PHP's OPcache, this makes Flow a very fast and efficient templating engine. Templates can be read from files, loaded from string arrays, or even from databases with relative ease.


The easiest way to install is by using Composer; the minimum composer.json configuration is:

    "require": {
        "flow/flow": "0.7.*"

Flow requires PHP 7 or newer.


Using Flow in your code is straight forward:

require 'path/to/vendor/autoload.php';

use Flow\Loader;
use Flow\Adapter\FileAdapter;

$flow = new Loader(
    new FileAdapter('path/to/templates'),
    new FileAdapter('path/to/cache')

try {
    $template = $flow->load('home.html');
        'data_1' => 'My first data',
        'data_2' => 'My second data',
} catch (\Exception $e) {
    // something went wrong!

The Loader constructor arguments are as follows:

  • mode: Recompilation mode.
  • source: Flow\Adapter object. See the section on loading templates from other sources near the bottom of this document.
  • target: Flow\Adapter object. See the section on loading templates from other sources near the bottom of this document.
  • helpers : Array of custom helpers.

The mode option can be one of the following:

  • Loader::RECOMPILE_NEVER: Never recompile an already compiled template.
  • Loader::RECOMPILE_NORMAL: Only recompile if the compiled template is older than the source file due to modifications.
  • Loader::RECOMPILE_ALWAYS: Always recompile whenever possible.

The default mode is Loader::RECOMPILE_NORMAL. If a template has never been compiled, or the compiled PHP file is missing, the Loader will compile it once regardless of what the current mode is.

In a typical development environment, the Loader::RECOMPILE_NORMAL mode should be used, while the Loader::RECOMPILE_NEVER mode should be used for production whenever possible. The Loader::RECOMPILE_ALWAYS mode is used only for internal debugging and testing purposes by the developers and should generally be avoided.

Two kinds of exceptions are thrown by Flow: SyntaxError for syntax errors, and RuntimeException for everything else.

Any reference to template files outside the source directory is considered to be an error.

Basic concepts

Flow uses {% and %} to delimit block tags. Block tags are used mainly for block declarations in template inheritance and control structures. Examples of block tags are block, for, and if. Some block tags may have a body segment. They're usually enclosed by a corresponding end<tag> tag. Flow uses {{ and }} to delimit output tags, {! and !} to delimit raw output tags, and {# and #} to delimit comments. Keywords and identifiers are case-sensitive.


Use {# and #} to delimit comments:

{# This is a comment. It will be ignored. #}

Comments may span multiple lines but cannot be nested; they will be completely removed from the resulting output.

Expression output

To output a literal, variable, or any kind of expression, use the opening {{ and the closing }} tags:

Hello, {{ username }}

{{ "Welcome back, " ~ username }}

{{ "Two plus two equals " ~ 2 + 2 }}

Raw expression output

To output a raw expression without doing any output escaping, use the opening {! and the closing !} tags:

The following will be HTML bold: {! "<b>bold text</b>" !}


There are several types of literals: numbers, strings, booleans, arrays, and null.


Numbers can be integers or floats:

{{ 42 }} and {{ 3.14 }}

Large numbers can be separated by underscores to make it more readable:

Price: {{ 12_000 | number_format }} USD

The exact placing of _ is insignificant, although the first character must be a digit; any _ character inside numbers will be removed. Numbers are translated into PHP numbers and thus are limited by how PHP handles numbers with regards to upper/lower limits and precision. Complex numeric and monetary operations should be done in PHP using the GMP extension or the bcmath extension instead.


Strings can either be double quoted or single quoted; both recognize escape sequence characters. There are no support for variable extrapolation. Use string concatenation instead:

{{ "This is a string " ~ 'This is also a string' }}

You can also join two or more strings or scalars using the join operator:

{{ "Welcome," .. user.name }}

The join operator uses a single space character to join strings together.


{{ true }} or {{ false }}

When printed or concatenated, true will be converted to 1 while false will be converted to an empty string. This behavior is consistent with the way PHP treats booleans in a string context.


{{ ["this", "is", "an", "array"][0] }}

Arrays are also hash tables just like in PHP:

{{ ["foo" => "bar", 'oof' => 'rab']['foo'] }}

Printing arrays will cause a PHP notice to be thrown; use the join helper:

{{ [1,2,3] | join(', ') }}


{{ null }}

When printed or concatenated, null will be converted to an empty string. This behavior is consistent with the way PHP treats nulls in a string context.


In addition to short-circuiting, boolean operators or and and returns one of their operands. This means you can, for example, do the following:

Status: {{ user.status or "default value" }}

Note that the strings '0' and '' are considered to be false. See the section on branching for more information. This behavior is consistent with the way PHP treats strings in a boolean context.

Comparison operators can take multiple operands:

{% if 1 <= x <= 10 %}
<p>x is between 1 and 10 inclusive.</p>
{% endif %}

Which is equivalent to:

{% if 1 <= x and x <= 10 %}
<p>x is between 1 and 10 inclusive.</p>
{% endif %}

The in operator works with arrays, iterators and plain objects:

{% if 1 in [1,2,3] %}
1 is definitely in 1,2,3
{% endif %}

{% if 1 not in [4,5,6] %}
1 is definitely not in 4,5,6
{% endif %}

For iterators and plain objects, the in operator first converts them using a simple (array) type conversion.

Use ~ (tilde) to concatenate between two or more scalars as strings:

{{ "Hello," ~ " World!" }}

String concatenation has a lower precedence than arithmetic operators:

{{ "1 + 1 = " ~ 1 + 1 ~ " and everything is OK again!" }}

Will yield

1 + 1 = 2 and everything is OK again!

Use .. (a double dot) to join two or more scalars as string using a single space character:

{{ "Welcome," .. user.name }}

String output, concatenations and joins coerce scalar values into strings.

Operator precedence

Below is a list of all operators in Flow sorted and listed according to their precedence in descending order:

  • Attribute access: . and [] for objects and arrays
  • Filter chaining: |
  • Arithmetic: unary - and +, %, /, *, -, +
  • Concatenation: .., ~
  • Comparison: !==, ===, ==, !=, <>, <, >, >=, <=
  • Conditional: in, not, and, or, xor
  • Ternary: ? :

You can group subexpressions in parentheses to override the precedence rule.

Attribute access


You can access an object's member variables or methods using the . operator:

{{ user.name }}

{{ user.get_full_name() }}

When calling an object's method, the parentheses are optional when there are no arguments passed. The full semantics of object attribute access are as follows:

For attribute access without parentheses, in order of priority:

  1. If the attribute is an accessible member variable, return its value.
  2. If the object implements __get, invoke and return its value.
  3. If the attribute is a callable method, call and return its value.
  4. If the object implements __call, invoke and return its value.
  5. Return null.

For attribute access with parentheses, in order of priority:

  1. If the attribute is a callable method, call and return its value.
  2. If the object implements __call, invoke and return its value.
  3. Return null.

You can always force a method call by using parentheses.


You can return an element of an array using either the . operator or the [ and ] operator:

{{ user.name }} is the same as {{ user['name'] }}

{{ users[0] }}

The . operator is more restrictive: only tokens of name type can be used as the attribute. Tokens of name type begins with an alphabet or an underscore and can only contain alphanumeric and underscore characters, just like PHP variables and function names.

One special attribute access rule for arrays is the ability to invoke closure functions stored in arrays:

$template = $flow->load('my_template.html');
    'user' => [
        'firstname' => 'Rasmus',
        'lastname'  => 'Lerdorf',
        'fullname'  => function($self) {
            return $self['firstname'] . ' ' .  $self['lastname'];

And call the fullname "method" in the template as follows:

{{ user.fullname }}

When invoked this way, the closure function will implicitly be passed the array it's in as the first argument. Extra arguments will be passed on to the closure function as the second and consecutive arguments. This rule lets you have arrays that behave not unlike objects: they can access other member values or functions in the array.

Dynamic attribute access

It's possible to dynamically access an object or array attributes:

{% assign attr = 'name' %}

Your name: {{ user[attr] }}


Helpers are simple functions you can use to test or modify values prior to use. There are two ways you can use them:

  • Using helpers as functions
  • Using helpers as filters

Except for a few exceptions, they are exchangeable.

Using helpers as functions

{{ upper(title) }}

You can chain helpers just like you can chain function calls in PHP:

{{ nl2br(upper(trim(my_data))) }}

Using helpers as filters

Use the | character to separate the data with the filter:

{{ title | upper }}

You can use multiple filters by chaining them with the | character. Using them this way is not unlike using pipes in Unix: the output of the previous filter is the input of the next one. For example, to trim, upper case and convert newlines to <br> tags (in that order), simply write:

{{ my_data | trim | upper | nl2br }}

Some built-in helpers accept additional parameters, delimited by parentheses and separated by commas, like so:

{{ "foo " | repeat(3) }}

Which is equivalent to the following:

{{ repeat("foo ", 3) }}

When using helpers as filters, be careful when mixing operators:

{{ 12_000 + 5_000 | number_format }}

Due to operator precedence, the above example is semantically equivalent to:

{{ 12_000 + (5_000 | number_format) }}

Which, when compiled to PHP, will output 12005 which is probably not what you'd expect. Either put the addition inside parentheses like so:

{{ (12_000 + 5_000) | number_format }}

Or use the helper as a function:

{{ number_format(12_000 + 5_000) }}

Built-in helpers

abs, bytes, capitalize, cycle, date, dump, e, escape, first, format, is_iterable, is_divisible_by, is_empty, is_even, is_odd, join, json_encode, keys, last, length, lower, nl2br, number_format, range, repeat, replace, strip_tags, title, trim, truncate, unescape, upper, url_encode, word_wrap.

Registering custom helpers

Registering custom helpers is straightforward:

use Flow\Loader;
use Flow\Adapter\FileAdapter;

$helpers = [
    'random' => function() { return 4; },
    'exclamation' => function($s = null) { return $s . '!'; },

$flow = new Loader(
    new FileAdapter('/path/to/templates'),
    new FileAdapter('/path/to/cache'),

try {
    $template = $flow->load('my_template.html');
} catch (\Exception $e) {
    // something went wrong!

You can use your custom helpers just like any other built-in helpers:

A random number: {{ random() }} is truly {{ "bizarre" | exclamation }}

When used as functions, the parentheses are necessary even if your helpers do not take any parameters. As a rule, when used as a filter, the input is passed on as the first argument to the helper. It's advisable to have a default value for every parameter in your custom helper.

Since built-in helpers and custom helpers share the same namespace, you can override built-in helpers with your own version although it's generally not recommended.


Use the if tag to branch. Use the optional elseif and else tags to have multiple branches:

{% if expression_1 %}
    expression 1 is true!
{% elseif expression_2 %}
    expression 2 is true!
{% elseif expression_3 %}
    expression 3 is true!
{% else %}
    nothing matches!
{% endif %}

Values considered to be false are false, null, 0, '0', '', and [] (empty array). This behavior is consistent with the way PHP treats data types in a boolean context. From experience, it's generally useful to have the string '0' be considered a false value: usually the data comes from a relational database which, in most drivers in PHP, integers in returned tuples are converted to strings. You can always use the strict === and !== comparison operators.

Inline if and unless statement modifiers

Apart from the standalone block tag version, the if tag is also available as a statement modifier. If you know Ruby or Perl, you might find this familiar:

{{ "this will be printed" if this_evaluates_to_true }}

The above is semantically equivalent to:

{%- if this_evaluates_to_true -%}
{{ "this will be printed" }}
{%- endif -%}

You can use any kind of boolean logic just as in the standard block tag version:

{{ "this will be printed" if not this_evaluates_to_false }}

Using the unless construct might be more natural for some cases. The following is equivalent to the above:

{{ "this will be printed" unless this_evaluates_to_false }}

Inline if and unless modifiers are available for output tags, break and continue tags, extends tags, parent tags, set tags, and include tags.

Ternary operator ?:

You can use the ternary operator if you need branching inside an expression:

{{ error ? '<p>' ~ error ~ '</p>' :  '<p>success!</p>' }}

The ternary operator has the lowest precedence in an expression.


Use the for tag to iterate through each element of an array or iterator. Use the optional else clause to implicitly branch if no iteration occurs:

{% for link in links %}
    <a href="{{ link.url }}">{{ link.title }}</a> {% else %}
{% else %}
    There are no links available.
{% endfor %}

Empty arrays or iterators, and values other than arrays or iterators will branch to the else clause.

You can also iterate as key and value pairs by using a comma:

{% for key, value in associative_array %}
    <p>{{ key }} = {{ value }}</p>
{% endfor %}

Both key and value in the example above are local to the iteration. They will retain their previous values, if any, once the iteration stops.

The special variable loop contains several useful attributes and is available for use inside the for block:

{% for user in users %}
    {{ user }}{{ ", " unless loop.last }}
{% endfor %}

If you have an ordinary loop variable, its value will temporarily be out of scope inside the for block.

The special loop variable has a few attributes:

  • loop.index: The zero-based index.
  • loop.count: The one-based index.
  • loop.first: Evaluates to true if the current iteration is the first.
  • loop.last: Evaluates to true if the current iteration is the last.
  • loop.parent: The parent iteration loop object if applicable.

Break and continue

You can use break and continue to break out of a loop and to skip to the next iteration, respectively. The following will print "1 2 3":

{% for i in [0,1,2,3,4,5] %}
    {% continue if i < 1 %}
    {{ i }}
    {% break if i > 2 %}
{% endfor %}


It is sometimes unavoidable to set values to variables and object or array attributes; use the assign construct:

{% assign fullname = user.firstname .. user.lastname %}

{% assign user.fullname = fullname %}

You can also use assign as a way to buffer output and store the result in a variable:

{% assign slogan %}
<p>This changes everything!</p>
{% endassign %}
{{ slogan }}

The scope of variables introduced by the assign construct is always local to its surrounding context.


Blocks are at the core of template inheritance:

{# this is in "parent_template.html" #}
{% block content %}
<p>Original content</p>
{% endblock %}

{# this is in "child_template.html" #}
{% extends "parent_template.html" %}
This will never be displayed!
{% block content %}
<p>This will be substituted to the parent template's "content" block</p>
{% endblock %}
This will never be displayed!

When child_template.html is loaded, it will yield:


<p>This will be substituted to the parent template</p>


Block inheritance works by replacing all blocks in the parent, or extended template, with the same blocks found in the child, or extending template, and using the parent template as the layout template; the child template layout is discarded. This works recursively upwards until there are no more templates to be extended. Two blocks in a template cannot have the same name. You can define blocks within another block, but not within macros.


The extends construct signals Flow to load and extend a template. Blocks defined in the current template will override blocks defined in extended templates:

{% extends "path/to/layout.html" %}

The template extension mechanism is fully dynamic with some caveats. You can use context variables or wrap it in conditionals just like any other statement:

{% extends layout if some_condition %}

You can also use the ternary operator:

{% extends some_condition ? custom_layout : "default_layout.html" %}

You cannot however use expressions and variables that are calculated inside the template before the extends tag. This is because the extends tag is the first thing a template will evaluate regardless of where its position is in the template. This is also why it's best to put your extends tags somewhere at the top of your templates. For example, the following will not work:

{% assign extend_template = true %}
{% extends "parent.html" if extend_template %}

The following will also not work because tpl is a value calculated inside the template before the extends tag:

{% assign tpl = "parent.html" %}
{% extends tpl %}

If however the extend_template or the tpl variables are context variables that already exist before the template loads, then the two examples above will work as expected.

It is a syntax error to declare more than one extends tag per template or to declare an extends tag anywhere but at the top level scope.

Parameterized template extension

Using the assign tag to override a context variable before extending a parent template will not work. This is because an extends tag is the first thing a template will evaluate regardless of where its position is in the template and extending a template will discard the current extending template's layout (i.e., everything outside block tags) in favor of the extended template's layout.

You can however pass an array to override a parent template's context when extending it. With a parent template:

{# this is in parent.html #}
{% if show %}
{% endif %}

And a child template:

{# this is in child.html #}
{% extends "parent.html" with ['show' => true] %}

Rendering the child template will produce:


Likewise, you can't use variables created using the assign tag inside the array parameter used with the extends tag. For example, the following will not work:

{% assign foo = "BAR" %}
{% extends "parent.html" with [some_string => foo] %}


By using the parent tag, you can include the parent block's contents inside the child block:

{% block child %}
    {% parent %}
{% endblock %}

Using the parent tag anywhere outside a block or inside a macro is a syntax error.


Macros are a great way to make flexible and reusable partial templates:

{% macro bolder(text) %}
<b>{{ text }}</b>
{% endmacro %}

To call them:

{% call bolder("this is great!") %}

All parameters are optional; they default to null while extra positional arguments passed are ignored. Flow lets you define a custom default value for each parameter:

{% macro bolder(text="this is a bold text!") %}
<b>{{ text }}</b>
{% endmacro %}

You can also use named arguments:

{% call bolder(text="this is a text") %}

Extra named arguments overwrite positional arguments with the same name and previous named arguments with the same name. The parentheses are optional only if there are no arguments passed. Parameters and variables declared inside macros with the assign construct are local to the macro and will cease to exist once the macro returns.

Macros are dynamically scoped. They inherit the calling context:

{% macro greet %}
<p>{{ "Hello," .. name }}</p>
{% endmacro %}

{% assign name = "Joe" %}

{% call greet %}

The above will print:

<p>Hello Joe</p>

The calling context is masked by the arguments and the default parameter values.

Macros are inherited by extending templates and at the same time overrides other macros with the same name in parent templates.

Defining macros inside blocks or other macros is a syntax error. Redefining macros in a template is also a syntax error.

Macro block and yield

You can call a macro with a block and yield inside the macro definition:

{% macro header %}
<header>{% yield %}</header>
{% endmacro %}

{% assign title = "Flow macro blocks is cool" %}

{% call header with %}<h1>{{ title or "This is the title" }}</h1>{% endcall %}

The above will result in:

<header><h1>Flow macro blocks is cool</h1></header>

It is possible to yield multiple times and to also provide context overrides:

{% macro header %}
<header>{% yield(title="Flow") %}</header>
{% endmacro %}

{% call header with %}<h1>{{ title }}</h1>{% endcall %}

Which will result in:


Importing macros

It's best to group macros in templates like you would functions in modules or classes. To use macros defined in another template, simply import them:

{% import "path/to/form_macros.html" as form %}

All imported macros must be aliased using the as keyword. To call an imported macro, simply prepend the macro name with the alias followed by a dot:

{% call form.text_input %}

Imported macros are inherited by extending templates and at the same time overrides other imported macros with the same alias and name pair in parent templates.

Decorating macros

You can decorate macros by importing them first:

{# this is in "macro_A.html" #}
{% macro emphasize(text) %}<b>{{ text }}</b>{% endmacro %}

{# this is in "macro_B.html" #}
{% import "macro_A.html" as A %}
{% macro emphasize(text) %}<i>{% call A.emphasize(text) %}</i>{% endmacro %}

{# this is in "template_C.html" #}
{% import "macro_B.html" as B %}
Emphasized text: {% call B.emphasize("this is pretty cool!") %}

The above when rendered will yield:

Emphasized text: <i><b>this is pretty cool!</b></i>


Use the include tag to include bits and pieces of templates in your template:

{% include "path/to/sidebar.html" if page.sidebar %}

This is useful for things like headers, sidebars and footers. Including non-existing or non-readable templates is a runtime error. Note that there are no mechanisms to prevent circular inclusion of templates, although there is a PHP runtime limit on recursion: either the allowed memory allocation size is reached, thereby producing a fatal runtime error, or the number of maximum nesting level is reached, if you're using xdebug.

Parameterized template inclusion

As with template extension, you can pass an array as the overriding context for the included template:

{% include "footer.html" with ['year' => current_year] %}

The array parameter will override any variables in the current context but only for the duration of the include.

Path resolution

Paths referenced in extends, include, and import tags can either be absolute from the specified source option when instantiating the loader object, or relative to the current template's directory.

Absolute paths

Absolute paths must begin with a / character like so:

{% include "/foo/bar.html" %}

In the example above, if the source directory is /var/www/templates, then the tag will try to include the template /var/www/templates/foo/bar.html regardless of what the current template's directory is.

Relative paths

Relative paths must not begin with a / character:

{% include "far.html" %}

In this example, if the source directory is /var/www/templates, and the current template's directory is boo, relative to the source, then the tag will try to include the template /var/www/templates/boo/far.html.

Path injection prevention

Flow throws a RuntimeException if you try to load any file that is outside the source directory.

Loading templates from other sources

Sometimes you need to load templates from a database or even string arrays. This is possible in Flow by simply passing an object of a class that implements the Flow\Adapter interface to the adapter option of the Loader constructor.

The Flow\Adapter interface declares five methods:

  • isReadable: Determines whether the path is readable or not.
  • lastModified: Returns the last modified time of the path.
  • getContents: Returns the contents of the given path.
  • putContents: Puts content in path and returns bytes written.
  • getStreamUrl: Returns the stream URL.

The source option given in the Loader constructor still determines if a template is valid; i.e., whether the template can logically be found in the source directory.

Below is an example of implementing a Flow adapter to string arrays:

use Flow\Loader;
use Flow\Adapter;
use Flow\Adapter\FileAdapter;

class ArrayAdapter implements Adapter
    static $templates = [
        'first.html' => 'First! {% include "second.html" %}',
        'second.html' => 'Second!',

    public function isReadable(string $path) : bool
        return isset(self::$templates[$path]);

    public function lastModified(string $path) : int
        return filemtime(__FILE__);

    public function getContents(string $path) : string
        return self::$templates[$path];

    public function putContents(string $path, string $contents) : int
        self::$templates[$path] = $contents;
        return strlen($contents);

    public function getStreamUrl(string $path) : string
        /* registering array stream wrapper storage is left as an exercise */
        return 'array://' . $path;

$flow = new Loader(
    new ArrayAdapter,
    new FileAdapter('/path/to/cache')

The above will compile the templates and render the following:

First! Second!

Controlling whitespace

When you're writing a template for a certain file format that is sensitive to whitespace, you can use {%- and -%} in place of the normal opening and closing block tags to suppress whitespaces before and after the block tags, respectively. You can use either one or both at the same time depending on your needs. The {{- and -}} delimiters are also available for expression output tags, while the {#- and -#} delimiters are available for comment tags.

The following is a demonstration of whitespace control:

    {%- for user in ["Alice", "Bob", "Charlie"] -%}
    <li>{{ user }}</li>
    {%- endfor -%}

Which will yield a compact


While the same example, this time without any white-space control:

    {% for user in ["Alice", "Bob", "Charlie"] %}
    <li>{{ user }}</li>
    {% endfor %}

Will yield the rather sparse


The semantics are as follows:

  • {%-, {{-, and {#- delimiters will remove all whitespace to their left up to but not including the first newline it encounters.

  • -%}, -}}, and -#} delimiters will remove all whitespace to their right up to and including the first newline it encounters.


Flow is released under the MIT License.


Flow is heavily based on the original Twig implementation by Armin Ronacher and subsequently influenced by Jinja2, Fabien Potencier's Twig fork, Python, and Ruby.