Null vs. False vs. 0 in PHP

Question

I am told that good developers can spot/utilize the difference between Null and False and 0 and all the other good "nothing" entities.
What is the difference, specifically in PHP? Does it have something to do with ===?

Answer 1

It's language specific, but in PHP :

Null means "nothing". The var has not been initialized.

False means "not true in a boolean context". Used to explicitly show you are dealing with logical issues.

0 is an int. Nothing to do with the rest above, used for mathematics.

Now, what is tricky, it's that in dynamic languages like PHP, all of them have a value in a boolean context, which (in PHP) is False.

If you test it with ==, it's testing the boolean value, so you will get equality. If you test it with ===, it will test the type, and you will get inequality.

So why are they useful ?

Well, look at the strrpos() function. It returns False if it did not found anything, but 0 if it has found something at the beginning of the string !

<?php
// pitfall :
if (strrpos("Hello World", "Hello")) { 
    // never exectuted
}

// smart move :
if (strrpos("Hello World", "Hello") !== False) {
    // that works !
}
?>

And of course, if you deal with states:

You want to make a difference between DebugMode = False (set to off), DebugMode = True (set to on) and DebugMode = Null (not set at all, will lead to hard debugging ;-)).