Shell
A shell is a program that provides an interface for users to interact with the operating system and execute commands. It acts as a command interpreter, taking input from the user, interpreting it, and then executing the appropriate actions.
PHP is able to relay commands to the OS using dedicated local functions, such as shell_exec(), exec(), system(), passthru() and the back-tick operator `. It is also able to access remote shells with the ssh:// protocol.
Shells provide an important access to the underlying OS: they are considered as a critical point of entry for security attacks.
<?php
$output = shell_exec('ls -lart');
echo "<pre>$output</pre>";
?>