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Error Identifier: instanceof.alwaysFalse

Every error reported by PHPStan has an error identifier. Here’s a list of all error identifiers. In PHPStan Pro you can see the error identifier next to each error and filter errors by their identifiers.

Code example #

<?php declare(strict_types = 1);

class Dog {}
class Cat {}

function doFoo(Dog $dog): void
{
	if ($dog instanceof Cat) { // ERROR: Instanceof between Dog and Cat will always evaluate to false.
		echo 'This is a cat';
	}
}

Why is it reported? #

The instanceof check will always evaluate to false because the type of the checked expression can never be an instance of the specified class. In this example, the parameter $dog is typed as Dog, which has no relationship to Cat (neither extends nor implements it), so the check is always false. This usually indicates a logic error, a wrong variable being checked, or a wrong class name.

How to fix it #

Check against the correct class:

 <?php declare(strict_types = 1);
 
 class Dog {}
 class Cat {}

 function doFoo(Dog $dog): void
 {
-	if ($dog instanceof Cat) {
-		echo 'This is a cat';
+	if ($dog instanceof Dog) {
+		echo 'This is a dog';
 	}
 }

Or widen the parameter type if the function should accept multiple types:

 <?php declare(strict_types = 1);
 
 class Dog {}
 class Cat {}

-function doFoo(Dog $dog): void
+function doFoo(Dog|Cat $animal): void
 {
-	if ($dog instanceof Cat) {
+	if ($animal instanceof Cat) {
 		echo 'This is a cat';
 	}
 }

How to ignore this error #

You can use the identifier instanceof.alwaysFalse to ignore this error using a comment:

// @phpstan-ignore instanceof.alwaysFalse
codeThatProducesTheError();

You can also use only the identifier key to ignore all errors of the same type in your configuration file in the ignoreErrors parameter:

parameters:
	ignoreErrors:
		-
			identifier: instanceof.alwaysFalse

Rules that report this error #

• PHPStan\Rules\Classes\ImpossibleInstanceOfRule [1]

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