If you need to execute a command without permission and could not execute it by ssh or install any extension, there is a way in Apache 1.3.x and PHP 4.
Create a file on cgi-bin directory, like this:
#!/usr/bin/php
<?
echo shell_exec('whoami');
?>
Don't forget to set the file you created the permission to execute it. Hence, call it from browser and you will se that this script will be executed by the shell user and not the user nobody (apache default user if running a PHP script).