What does "return $this" mean?

$this means the current object, the one the method is currently being run on. By returning $this a reference to the object the method is working gets sent back to the calling function.

So anyone doing

 $foo2 = $foo->SetOptions($bar);

$foo2 now refers to $foo also.

you just can create a function chain

class My_class
{

        public function method1($param)
        {
                /*
                 * logic here
                 */

                return $this;
        }

        public function method2($param)
        {
                /*
                 * logic here
                 */

                return $this;
        }

        public function method3($param)
        {
                /*
                 * logic here
                 */

                return $this;
        }

}

so you can use this

            My_class obj = new My_class();

            $return = obj->method1($param)->method2($param)->method3($param);

This way of coding is called fluent interface. return $this returns the current object, so you can write code like this:

$object
  ->function1()
  ->function2()
  ->function3()
  ;

instead of:

$object->function1();
$object->function2();
$object->function3();

This will return the instance this method is called on. This usually done for achieving fluent interfaces so you can call stuff like:

CoolClass::factory('hello')->setOptions(array('coolness' => 5))->sayHello();

Where both setOptions and sayHello would be called on the same object.