instanceof.alwaysFalse - Phpstan

Code example

php

<?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:

diff-php

 <?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:

diff-php

 <?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';
 	}
 }