What is the difference between NULL and null?

Under lines , there are no differences. NULL is case insensitive - not case-sensitive. In the documentation this is said:

  

There is only one null value, and this is the constant case insensitive NULL.

The same goes for TRUE and FALSE . As also said in documentation , both are case insensitive :

  

To specify a literal Boolean, use the TRUE or FALSE constants. Both are case insensitive .

So you can use which one is more convenient, or which you have more familiarity with.

PSR's comments on this. Although they are just a style guide and not rules to enforce, they recommend that:

  

PHP keywords must be in lowercase . PHP , false and null constants should be written in lowercase .

We also have a question answered about this in our Big Brother SO .