PHP - insert unique identifier to file - steganography

id.php

<?php

function sendfile($file,$identifier,$id_length) {
	if (file_exists($file)) {
		$filesize = filesize($file)+$id_length;
		header('Content-Description: File Transfer');
		header('Content-Type: application/octet-stream');
		header('Content-Disposition: attachment; filename='.basename($file));
		header('Content-Transfer-Encoding: binary');
		header('Expires: 0');
		header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
		header('Pragma: public');
		header('Content-Length: ' . $filesize);
		ob_clean();
		flush();
		readfile($file);
		echo $identifier;
		exit;
	}
	else {
		echo "FILE NOT FOUND! :(";
	}
}

sendfile("somefile","8digitid",8);

?>