Parallel testing for PHP

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v7.5.0 2024-06-12 13:02 UTC



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The objective of ParaTest is to support parallel testing in PHPUnit. Provided you have well-written PHPUnit tests, you can drop paratest in your project and start using it with no additional bootstrap or configurations!


  • Zero configuration. After the installation, run with vendor/bin/paratest to parallelize by TestCase or vendor/bin/paratest --functional to parallelize by Test. That's it!
  • Code Coverage report combining. Run your tests in N parallel processes and all the code coverage output will be combined into one report.


To install with composer run the following command:

composer require --dev brianium/paratest


Only the latest version of PHPUnit is supported, and thus only the latest version of ParaTest is actively maintained.

This is because of the following reasons:

  1. To reduce bugs, code duplication and incompatibilities with PHPUnit, from version 5 ParaTest heavily relies on PHPUnit @internal classes
  2. The fast pace both PHP and PHPUnit have taken recently adds too much maintenance burden, which we can only afford for the latest versions to stay up-to-date


After installation, the binary can be found at vendor/bin/paratest. Run it with --help option to see a complete list of the available options.

Test token

The TEST_TOKEN environment variable is guaranteed to have a value that is different from every other currently running test. This is useful to e.g. use a different database for each test:

if (getenv('TEST_TOKEN') !== false) {  // Using ParaTest
    $dbname = 'testdb_' . getenv('TEST_TOKEN');
} else {
    $dbname = 'testdb';

A UNIQUE_TEST_TOKEN environment variable is also available and guaranteed to have a value that is unique both per run and per process.

Code coverage

The cache is always warmed up by ParaTest before executing the test suite.


If you have installed pcov but need to enable it only while running tests, you have to pass thru the needed PHP binary option:

php -d pcov.enabled=1 vendor/bin/paratest --passthru-php="'-d' 'pcov.enabled=1'"


If you have xDebug installed, activating it by the environment variable is enough to have it running even in the subprocesses:

XDEBUG_MODE=coverage vendor/bin/paratest

Initial setup for all tests

Because ParaTest runs multiple processes in parallel, each with their own instance of the PHP interpreter, techniques used to perform an initialization step exactly once for each test work different from PHPUnit. The following pattern will not work as expected - run the initialization exactly once - and instead run the initialization once per process:

private static bool $initialized = false;

public function setUp(): void
    if (! self::$initialized) {
         self::$initialized = true;

This is because static variables persist during the execution of a single process. In parallel testing each process has a separate instance of $initialized. You can use the following pattern to ensure your initialization runs exactly once for the entire test invocation:

static bool $initialized = false;

public function setUp(): void
    if (! self::$initialized) {
        // We utilize the filesystem as shared mutable state to coordinate between processes
        $lockFile = fopen('/tmp/test-initialization-lock-file', 'r');

        // Attempt to get an exclusive lock - first process wins
        if (flock($lockFile, LOCK_EX | LOCK_NB)) {
            // Since we are the single process that has an exclusive lock, we run the initialization
        } else {
            // If no exclusive lock is available, block until the first process is done with initialization
            flock($lockFile, LOCK_SH);

        self::$initialized = true;


If you run into problems with paratest, try to get more information about the issue by enabling debug output via --verbose --debug.

When a sub-process fails, the originating command is given in the output and can then be copy-pasted in the terminal to be run and debugged. All internal commands run with --printer [...]\NullPhpunitPrinter which silence the original PHPUnit output: during a debugging run remove that option to restore the output and see what PHPUnit is doing.


  1. Constants, static methods, static variables and everything exposed by test classes consumed by other test classes (including Reflection) are not supported. This is due to a limitation of the current implementation of WrapperRunner and how PHPUnit searches for classes. The fix is to put shared code into classes which are not tests themselves.

Integration with PHPStorm

ParaTest provides a dedicated binary to work with PHPStorm; follow these steps to have ParaTest working within it:

  1. Be sure you have PHPUnit already configured in PHPStorm:
  2. Go to Run -> Edit configurations...
  3. Select Add new Configuration, select the PHPUnit type and name it ParaTest
  4. In the Command Line -> Interpreter options add ./vendor/bin/paratest_for_phpstorm
  5. Any additional ParaTest options you want to pass to ParaTest should go within the Test runner -> Test runner options section

You should now have a ParaTest run within your configurations list. It should natively work with the Rerun failed tests and Toggle auto-test buttons of the Run overlay.

Run with Coverage

Coverage with one of the available coverage engines must already be configured in PHPStorm and working when running tests sequentially in order for the helper binary to correctly handle code coverage

For Contributors: testing ParaTest itself

Before creating a Pull Request be sure to run all the necessary checks with make command.