Try this link at PHP Manual
or just read the following from the link above:

NULL

The special NULL value represents that a variable has no value. NULL is the only possible value of type NULL.

Note: The null type was introduced in PHP 4.

A variable is considered to be NULL if

*

  it has been assigned the constant NULL.
*

  it has not been set to any value yet.
*

  it has been unset().

Syntax

There is only one value of type NULL, and that is the case-insensitive keyword NULL

Hope that helps you out?

    If null means an empty variable then yea 😃 thanx

      Well it's not an empty variable, it's an empty anything. For example, some functions can be expected to return NULL as well. 'NULL' is just a keyword in PHP meaning "nothing".

      From the dictionary:

      null
      –adjective
      1. without value, effect, consequence, or significance.

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