Опять же - ищем проблемы там, где их нет. Чем nohup не угодил - я не знаю, но для любителей есть и следующее.

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        # child gets instantiated, note that child must NOT use any of the parent resources
        $pid = pcntl_fork();
        if ($pid < 0) return false; # failed
        if ($pid != 0) return true; # succeeded

        # ignore child signals and detach from controlling pty
        pcntl_signal(SIGCHLD, SIG_IGN);
        posix_setsid();

        # fork second time
        $pid = pcntl_fork();
        if ($pid != 0) exit; # failed to fork second time or master

        # second level slave here, detached from the master pty, we still detach it again to be safe
        posix_setsid();
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