PHP supports one execution operator: backticks (``). Note that these are not single-quotes! PHP will attempt to execute the contents of the backticks as a shell command; the output will be returned (i.e., it won't simply be dumped to output; it can be assigned to a variable).
$output = `ls -al`; echo "<pre>$output</pre>";
See also system(), passthru(), exec(), popen(), and escapeshellcmd().