I cannot reproduce but I didn't try on Windows/PowerShell, help wanted from someone that has the issue. Otherwise we'll have to close.

I have same error, on Windows 7, PHP 7.2.8
Steps to reproduce:

composer create-project symfony/skeleton testunit
cd testunit
composer require --dev symfony/phpunit-bridge
php bin\phpunit

Result:

INFO: Could not find files for the given pattern(s).
Installing phpunit/phpunit (6.5.14)
Plugins have been disabled.
  - Installing phpunit/phpunit (6.5.14): Loading from cache
Created project in phpunit-6.5
composer.json has been updated

Warning: proc_open(): CreateProcess failed, error code - 2 in E:\web\_hosts\test.local\symfony\testunit\vendor\symfony\phpunit-bridge\bin\simple-phpunit.php on line 131
Warning: proc_close() expects parameter 1 to be resource, boolean given in E:\web\_hosts\test.local\symfony\testunit\vendor\symfony\phpunit-bridge\bin\simple-phpunit.php on line 131
Warning: require(E:\web\_hosts\test.local\symfony\testunit\bin\.phpunit\phpunit-6.5/vendor/autoload.php): failed to open stream: No such file or directory in E:\web\_hosts\test.local\symfony\testunit\bin\.phpunit\phpunit-6.5\phpunit on line 4
Fatal error: require(): Failed opening required 'E:\web\_hosts\test.local\symfony\testunit\bin\.phpunit\phpunit-6.5/vendor/autoload.php' (include_path='.;C:\php\pear') in E:\web\_hosts\test.local\symfony\testunit\bin\.phpunit\phpunit-6.5\phpunit on line 4

Looks like problem in bypass_shell option in vendor\symfony\phpunit-bridge\bin\simple-phpunit.php on line 131. Composer runs from composer.bat file and located in some directory on my PC. That directory added into PATH env to execute composer anywhere, but proc_open with bypass_shell option don't know about this directory. When i set this option to false, phpunit-bridge working fine.

simple-phpunit use different exec ways, that probably a part of problem. Initially simple-phpunit trying to locate composer by backtick operator (that use cmd.exe and PATH env) on line 94 after that trying run composer by proc_open and bypassing cmd.exe env variables.

Same problem on windows 10, PHP7.2
It almost works with a WSL (Windows Subsystem Linux)

@misterx thanks for the hints. Would you be able to submit a patch fixing the issue?

Looks like problem in bypass_shell option in vendor\symfony\phpunit-bridge\bin\simple-phpunit.php on line 131. Composer runs from composer.bat file and located in some directory on my PC. That directory added into PATH env to execute composer anywhere, but proc_open with bypass_shell option don't know about this directory. When i set this option to false, phpunit-bridge working fine.

simple-phpunit use different exec ways, that probably a part of problem. Initially simple-phpunit trying to locate composer by backtick operator (that use cmd.exe and PATH env) on line 94 after that trying run composer by proc_open and bypassing cmd.exe env variables.

c'est chaud je cherche a faire des test unitaire via gitlab ci cd avec docker vendor/bin/phpunit ne se lance pas

Hi zapo1234 , j'ai le même soucis impocis de jouer les test unitaire via gitlab ci cd avec docker. As tu trouvé une solution depuis lors svp?

Error 404 - French not found ;)
I also get following on composer update:

In BinaryInstaller.php line 207:
                                                                                                                
  file_put_contents(/var/www/html/vendor/bin/simple-phpunit): failed to open stream: No such file or directory  

Is there some solution?

Cheers