this keyword

This is a keyword that you'll see alot of in, not just php, but nearly all object-oriented programming languages.

The this-keyword is basically a reference to the current object. Here is an example:

Class Car
{
   public $manufacturer;    //this is a variable of the Car-class and will store the manufacturer-name for a Car-object.
   public $model; //this is also a variable of the Car-class and will store the model-name for a Car-object. 

   function __construct( $manufacturer, $model ) {
        $this->manufacturer = $manufacturer;
        $this->model = $model;
    }
};

$car1 = new Car('Volvo', 'XC90');
echo $car1->manufacturer;
echo $car1->model;

$car2 = new Car('Ferrari', 'F50');
echo $car2->manufacturer;
echo $car2->model; 

So in the above code you have a class that has 2 attributes: $manufacturer and $model. When you create a new Car-object, these 2 properties are given to the object. So you can read $this as "The current object".

So where it says $this->model = $model; you can read as "The current objects model is = the variable $model". You can then grab these properties by referencing the object (here $car1 or $car2) and then its' property-name.

From php.net manual:

The pseudo-variable $this is available when a method is called from within an object context. $this is a reference to the calling object (usually the object to which the method belongs, but possibly another object, if the method is called statically from the context of a secondary object).

http://www.php.net/manual/en/language.oop5.basic.php

Hope this helps