What are the security implications of using system commands in PHP scripts to execute files?

Using system commands in PHP scripts to execute files can pose a significant security risk as it opens up the possibility of command injection attacks. To mitigate this risk, it is recommended to use PHP functions like `exec()`, `shell_exec()`, or `proc_open()` with proper input validation and sanitization to prevent malicious commands from being executed.

$filename = "file.txt";
$escaped_filename = escapeshellarg($filename);
$output = shell_exec("cat " . $escaped_filename);
echo $output;