function is_class_a($a, $b)
{
    return $a == $b || is_subclass_of($a, $b);
}

<?php

class a{}

class b extends a{}

$r = new ReflectionClass( 'b' );

echo "class b "
    , (( $r->isSubclassOf( new ReflectionClass( 'a' ) ) ) ? "is" : "is not")
    , " a subclass of a";