validate_int.php

$list_of_ids = '1,2,3,4,5,6';
if ( cg_validate_int( $list_of_ids ) ) {
	echo 'Huzzah! We have valide integers!';
}

/**
 * Allows us to pass either an array of or a single intger and validate that they are integers
 * 
 * @param  int|string $list_of_ids Pass either a single integer or a comma separated list
 *
 * @return bool
 */
function cg_validate_int( $list_of_ids ) {

    //loop through the split list
    foreach (explode( ',', $list_of_ids ) as $id) {

        //validate the int by converting back and checking against the original string
        //zero will handle properly with this approach
        if(strval(intval($id)) !== $id) {

            //short circuit if you find a bad value, no need to continue
            return false;
        }
    }
    
    //if you got here, the list is good
    return true;
}