exec() system() help!!! dynamic libraries problem?

I have php-4.0.0 compiled as an Apache module on RedHat 6.2, i386. It behaves quite strange when I try to use exec("command", $result_array ) from the script. Some commands are executed, some aren't. I figured that the only difference between successful and unsuccessful ones is use (absence of use) of shared libraries. Did any body have the same problem? What are the possible workarounds other than translating C code that I'm trying to exec() into PHP? Thanks, Sergiy.