I guess you are not aquainted with OOP (Object Oriented Programming - http://www.go4expert.com/showthread.php?t=436), this always refers to the “owner” of the function we're executing, or rather, to the object that a function is a method of.

In you example, $mail contains the instance of some Class/Object, and mail is a method of the Class/Object, to call the method we use the arrow (->) operator.