src/Reflection/ReflectionClass.php
public function isSubclassOf(string $className) : bool
    {
        return in_array(
            ltrim($className, '\\'),
            array_map(
                static function (self $reflectionClass) : string {
                    return $reflectionClass->getName();
                },
                array_slice(array_reverse($this->getInheritanceClassHierarchy()), 1)
            ),
            true
        );
    }
public function isSubclassOf(string $className) : bool
    {
        return in_array(
            ltrim($className, '\\'),
            array_map(
                static function (self $reflectionClass) : string {
                    return $reflectionClass->getName();
                },
                array_slice(array_reverse($this->getInheritanceClassHierarchy()), 1)
            ),
            true
        );
    }
Checks whether the given class string is a subclass of this class.

@link https://php.net/manual/en/reflectionclass.isinstance.php