Operators
PHP Manual

Execution Operators

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). Use of the backtick operator is identical to shell_exec.

<?php
$output = `ls -al`;
echo "<pre>$output</pre>";
?>

Note:

The backtick operator is disabled when safe mode is enabled or shell_exec is disabled.

Note:

Unlike some other languages, backticks have no special meaning within double-quoted strings.

See also the manual section on Program Execution functions, popen proc_open, and Using PHP from the commandline.