Difference between NULL and null in PHP

Question

Is there a difference between NULL and null in PHP? Sometimes they seem to be interchangeable and sometimes not.

edit: for some reason when I read the documentation linked to in the answer (before posting this question) I read it as "case sensitive" instead of "case insensitive" which was the whole reason I posted this question in the first place...

Answer 1

Null is case insensitive.

From the documentation:

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

Answer 2

Usually in programming 'null' is empty varibale of an any type while no memory for it is allocated. In php I gues too.

Foo f; // is null
f = new Foo(); // 'new' is a operator, function for memory allocating

'SQL NULL' is the sutiation when value for a column is not yet assigned. NOT NULL attribute is not set and no default value is provided.

INSERT INTO t(ID,value) VALUES ('{123}', NULL);

In general, this concepts are the same. 'Not yet assisgned value'. But in implementation, 'null' is empty value, but DbNull is special class which represents SQL NULL.