is_numeric($str) for database queries

Question

Basically my question is, if I have a number string, and i'm going to compare it to the database, is that a secure/safe way to check. Or should I just escape my number variables aswell as my strings (like I already do)?

Example:

<?php
    $id = $_POST['id'];
    if(is_numeric($id)){
        //database connectivity.    
    }
?>

Answer 1

"Database" is pretty general, so I'll assume you're using MySQL. It's safe so long as when you insert $id into the query it is escaped (either with mysql_real_escape_string or preferably a prepared statement).

Answer 2

"Select from Tbl where Id = '". (int)$_POST['id'] ."'"