What is the equivalent of Python's None singleton in PHP?

Question

How do you represent the concept of "undefined" in PHP?

SQL has "NULL", Python has "None", what is the equivalent in PHP?

[updated]

Tanks everybody for the enlightening discussion (cHao, had you written a response rather than a comment I would choose yours).

Answer 1

Php has NULL too

Taken from the docs

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

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().