Here’s a thought exercise, can you tell exactly under what conditions is the following statement true, and when is it false?
$var = get_var();
if( ! 'some value' === $var && ! 'some other value' === $var ) {
echo "Do the thing";
}At first, you might think that the condition will only execute, if $var turns out not to be 'some value'and 'some other value' if that was you, take another look, then read on 🙂
The real answer is - “Do the thing” is only going to run when $var is false because ! "Do the thing" is evaluated first, and it’s cast to a boolean so that PHP can invert it, resulting in false.
In other words, the condition above, is the same as:
if( false === $var ) {
echo "Do the thing";
}Here’s another example to illustrate how the exclamation point ( ! ) works in PHP conditions:
// Arrays not equal
> ['a', 'b'] == ['a', 'b', 'c']
bool(false)
// Arrays are equal
> ['a', 'b'] == ['a', 'b']
bool(true)
// Array is cast to false by !
> ! ['a', 'b']
bool(false)
// Another example of array cast to false
> ! ['a', 'b', 'c']
bool(false)
// This will cast only 1 array and compare false to the other array
> ! ['a', 'b'] == ['a', 'b', 'c']
bool(false)
// Same as above
> ! ['a', 'b'] == ['a', 'b']
bool(false)The lesson here is to be explicit - if you want to negate a comparison - don’t try to save on file size by omitting parenthesis. Or better yet, use strong inequality check directly:
// As we established, this is a bug:
! 'value' === $var
// But instead of this
! ( 'value' === $var )
// Do this:
'value' !== $var