PHP控制台命令不会在进程结束时停止

I get a strange PHP bug on a PHP 5.6 / Symfony 2.7 project, running on a CentOS6 server through Apache.

I have a Symfony console command running as a service which launches some other console commands every 2 seconds. I use the Symfony Process component to launch the sub-processes and have timeout management. And everything is done to avoid to launch parallel processes from the main command.

The issue I have is that sometimes php console commands doesn't stop after finishing their process. Which means that if I launch by hand the commands, everything runs correctly on the PHP side but I don't get the hand back on the console after the PHP statements finished, unless I use Ctrl+C.

The issue was happening a lot of times when the PHP version was 5.5, but now with PHP 5.6 it (only) happens randomly. When it happens, I can see a lot of stucked php sub-processes, probably launched by the main command.

I just can't find any explanation since php commands doesn't raise any error. It's just that the console get stuck and wait for something to finish.

Has anybody a possible solution to this issue?