booleanNot.exprNotBoolean

Code example

<?php declare(strict_types = 1);

$string = 'str';

if (!$string) {
	// ...
}

Why is it reported?

The negation operator ! is being applied to a non-boolean value. PHP will implicitly cast the value to a boolean using its type juggling rules before negating it. This implicit conversion can lead to unexpected behaviour, for example !0 is true, !'' is true, and !'0' is also true.

This rule is part of phpstan-strict-rules and enforces that only boolean values are used with the ! operator, making the code's intent explicit.

In the example above, $string is of type string, not bool, so negating it relies on PHP's loose type coercion.

How to fix it

Use an explicit comparison instead of relying on type juggling:

 <?php declare(strict_types = 1);

 $string = 'str';

-if (!$string) {
+if ($string === '') {
 	// ...
 }

Or cast the value to boolean explicitly before negating:

 <?php declare(strict_types = 1);

 $string = 'str';

-if (!$string) {
+if (!((bool) $string)) {
 	// ...
 }