Do not wait for a parallel process to finish

@dankocherga, for me it seems the module doesn't actually work withouth
this commmit.

According to php.net documentation:

    If a program is started with this function, in order for it to
    continue running in the background, the output of the program must be
    redirected to a file or another output stream. Failing to do so will
    cause PHP to hang until the execution of the program ends.