When checking instanceof against a subclass of the class in question, it will return true.

<?php

class Foo {

    public $foobar = 'Foo';
    
    public function test() {
        echo $this->foobar . "\n";
    }

}

class Bar extends Foo {

    public $foobar = 'Bar';

}

$a = new Foo();
$b = new Bar();

echo "use of test() method\n";
$a->test();
$b->test();

echo "instanceof Foo\n";
var_dump($a instanceof Foo); // TRUE
var_dump($b instanceof Foo); // TRUE

echo "instanceof Bar\n";
var_dump($a instanceof Bar); // FALSE
var_dump($b instanceof Bar); // TRUE

echo "subclass of Foo\n";
var_dump(is_subclass_of($a, 'Foo')); // FALSE
var_dump(is_subclass_of($b, 'Foo')); // TRUE

echo "subclass of Bar\n";
var_dump(is_subclass_of($a, 'Bar')); // FALSE
var_dump(is_subclass_of($b, 'Bar')); // FALSE

?>

<?php
$obj = new A();
if ($obj instanceof A) {
   echo 'A';
}
?>

To be able to verify whether a PHP variable is an instantiated object of a certain class we use instanceof.