Question

Possible Duplicate:
Why do some experienced programmers write comparisons with the value before the variable?

I am just curious about this: in most frameworks/opensource projects I have studied, I often seen code like this…

<?php

if (null === self::$_instance) {
    self::$_instance = new self();
}

In particular this line…

if (null === self::$_instance) {

Why use null in the first argument of the if statement instead of the other way around?…

if (self::$_instance === null) {

I realize there is probably no performance increase or anything like that. Is this just a preference or is it some kind of coding standard I have overlooked?

Answer 1

It prevents you from accidentally assigning the value to a variable, especially when only using loose type comparison (==):

if (self::$_instance = NULL) { … } // WHOOPS!, self::$_instance is now NULL

This style of conditions is often called yoda conditions. Performance wise there is no difference, both statements are equivalent.