Apache and PHP: Execution of unix commands.

Hi, I've installed LiNUX 7.2, no patches are installed. The Apache server is up and running with the PHP4 module. I've installed some php pages to manage our servers, but some shell commands must be run with the exec system call. Note that I've did not make some changes on the default configuration of the httpd.conf file. According to what's going on, scripts files are not excuted. Is there some modification into configurations files ? Somes updates of PHP ? Thanks for your help. Pascal MiQUET