Description:
------------
When doing a external function call using either passthru(), 
proc_open() or system(), the external process hangs most of the time. 
Sometimes it works. Only appears when run through mod_php / apache2. 
Calling the script in the shell with CLI always works. 

ps shows:
11606 ?        Sl     0:00 /usr/bin/pdftk 

No errors in any logs, no segfaults. top

Please Note:
- doing the exact same call in a Perl script running in Apache always 
works.
- calling the perl script which does the funtion call from within php 
will crash again with ps (passthru example):

11674 ?        S      0:00 sh -c cd '/data/usr/inauris.com/tmp' ; 
/data/usr/inauris.com/tmp/test3.pl
11675 ?        S      0:00 /usr/bin/perl -w 
/data/usr/inauris.com/tmp/test3.pl
11676 ?        Sl     0:00 /usr/bin/pdftk 
/data/usr/inauris.com/management/home/app/pdf...

Apache Version: 2.2.11, Worker MPM

Reproduce code:
---------------
passthru ("/usr/bin/pdftk /data/testin.pdf fill_form /data/workdata output /data/testout.pdf flatten");

Expected result:
----------------
exec & return

Actual result:
--------------
hang