PHP Session ID Format Verification

sessidfverify.php

<?php

# INFO ABOUT PHP SESSION ID COOKIE
define('SERVER_SESS_ID', session_id());
define('SERVER_SESS_CHAR', '26');

# PHP SESSION ID FORMAT VERIFICATION
if(isset($_COOKIE[SERVER_SESS_ID])) {
    if (!preg_match('/^[a-z0-9]{'.SERVER_SESS_CHAR.'}$/', $_COOKIE[SERVER_SESS_ID])) {
        setrawcookie(SERVER_SESS_ID, '', 1, '/', null, null, true);
        exit('session validation error - try refreshing');
    }
}