PHP if statement null === $variable: what for?

Question

Often, I see in if statements for php something like this:

if (null === $variable) {
    // do stuff
}

What does it do and what is it for?

EDIT: I totally get that it is a comparison operator, I just wonder why not $variable === null.

Those aren't assignments, they are checks to see if a variable is null or not — Andrew Li, Aug 28 '16 at 07:51
Please find out in the documentation the difference between `=`, `==` and `===` — D.Dimitrioglo, Aug 28 '16 at 07:52
@AndrewL., sorry my mistake corrected, but why not to check like this `$variable === null`? why it's the other way around? — Dima Dz, Aug 28 '16 at 07:54
determines whether equal value are identical and of the same type — Dev. Joel, Aug 28 '16 at 07:56

T.J. Crowder · Accepted Answer · 2016-08-28T07:55:25.930

3

It's not an assignment, it's a comparison for equality. It determines if the variable $variable contains the value null.

More in the documentation:

why not to check $variable === null

Some people like to use the form with the constant on the left (a "Yoda condition", it is called) so that if they have a typo and only type a single =, it causes a syntax error rather than doing an assignment.

edited Aug 28 '16 at 07:55

answered Aug 28 '16 at 07:52

T.J. Crowder

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sorry modified the question, but why not to check `$variable === null`? – Dima Dz Aug 28 '16 at 07:53
Not the same language, but same reason: http://stackoverflow.com/questions/271561/why-does-one-often-see-null-variable-instead-of-variable-null-in-c – Andrew Li Aug 28 '16 at 07:55
@DimaDz: I've added a note above about that. – T.J. Crowder Aug 28 '16 at 07:55

Peter Chaula · Answer 2 · 2016-08-30T20:58:40.323

That is the Yoda style usually used as a trick by programmers to prevent accidental assignments which always give some silent bugs.

Example:

  var a = dosomething();

  if(a = null){
      //more here
   }

Note that the if block will always not execute regardless of the result of doSomething method since we assign then check for equality. This assignment nullifies the possibly non-deterministic nature of doSomething

PHP if statement null === $variable: what for?

2 Answers2