exec() and memory consumption

Hi all, I am trying to execute a shell command from PHP. This command consumes a lot of memory and execution time, so I changed max_execution_time = 120 and memory_limit = 256000000 in my php.ini. Still, under PHP, the command stops after some time (every time at the same point). When I execute the command from the command line, it finishes okay. By the way, I am executing : 'fop <file.xml> <file.xsl> <file.pdf> | logger' so I can monitor the output in /var/log/messages. Fop is the (Java) PDF generator from the Apache XML group. Can anyone tell me if there is a built in memory limit in PHP (e.g. 16 meg), or does anyone have had the same phenomenon when running something similar ? Any comments are greatly appreciated... Thx, Miguel