What are the potential security risks of using shell commands in PHP?

Using shell commands in PHP can introduce security risks such as command injection attacks, where an attacker can manipulate input to execute arbitrary commands on the server. To mitigate this risk, it is essential to properly sanitize and validate user input before using it in shell commands.

$user_input = $_POST['user_input'];
$validated_input = escapeshellarg($user_input);

// Use the validated input in the shell command
$output = shell_exec("ls " . $validated_input);