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>";
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See also system(), passthru(), exec(), popen(), and escapeshellcmd().