Assigning NULL to a variable in PHP: what does that do?

Question

In maintaining code, I'm encountering loops, where at the end of the loop several variables are set to NULL like so: $var = NULL;. From what I understand in the manual, NULL is meant mostly as something to compare against in PHP code. Since NULL has no type and is not a string or number, outputting it makes no sense.

I unfortunately cannot provide an example, but I think the NULL values are being written to a file in our code. My question is: does $var have a value after the assignment, and will echoing/writing it produce output?

EDIT: I have read the PHP manual entry on NULL. There is no need to post this: http://php.net/manual/en/language.types.null.php in a comment or answer, or top downvote me for not having RTM. Thank you!

Answer 1

Null in PHP means a variable were no value was assigned.

http://php.net/manual/en/language.types.null.php

Answer 2

null is pretty much just like any other value in PHP (actually, it's also a different data type than string, int, etc.).

However, there is one important difference: isset($var) checks for the var to exist and have a non-null value.

If you plan to read the variable ever again before assigning a new value, unset() is the wrong way to do but assigning null is perfectly fine:

php > $a = null;
php > if($a) echo 'x';
php > unset($a);
php > if($a) echo 'x';
Notice: Undefined variable: a in php shell code on line 1
php >

As you can see, unset() actually deletes the variable, just like it never existed, while assigning null sets it to a specific value (and creates the variable if necessary).

A useful use-case of null is in default arguments when you want to know if it was provided or not and empty strings, zero, etc. are valid, too:

function foo($bar = null) {
    if($bar === null) { ... }
}