how to set file permission 0555 using ftp_chmod in php

Question

How to set file permission 0555 using ftp_chmod?

<?php

$ftp_server = "example.com";
$ftp_conn = ftp_connect($ftp_server) or die("Could not connect to $ftp_server");
$login = ftp_login($ftp_conn, "user", "password");

$file = "path/index.php";
ftp_chmod($ftp_conn, 0555, $file)
?>

using this script i can not change file permission to 0555!! using this script i can change 0666,0777 but i cant not change to 0555

output of above script is 0655!!