Execute a shell command using thttpd-php

Hello, I'm quite new to PHP language and although I got into it pretty fast I'm having some trouble executing system commands using php. I'm using thttpd-php_2.21b-2_mipsel in a Linksys router and I can only execute system commands like "ls" and "cd" from the PHP. I tried using the different available functions (exec(), shell_exec(), system(), passthru() and backsticks) but none of them works. I can't even run scripts! I'm working on a web application that will be used to configure the router and I need to use a lot of other commands from the PHP, so right now I'm quite stucked. I tried to find help online and offline but wasn't able to find any. I just hope you guys can help me! Thanks, Nelson Carreira