What are the security implications of using shell commands in PHP scripts and how can they be mitigated?

Using shell commands in PHP scripts can pose security risks such as command injection attacks if user input is not properly sanitized. To mitigate these risks, it is important to validate and sanitize user input before using it in shell commands. Additionally, using PHP's built-in functions like escapeshellarg() or escapeshellcmd() can help prevent command injection vulnerabilities.

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

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