First use of the :: is to envoke a method:
Person::speak();
Hello, how are you?
Second use of the :: is to refer to static properties together with the 'self' keyword:
My name is self::$name
My name is Ramon Hitzeroth
Third user of the :: is to refer to static properties from another class:
In New_Person, the message from the Person class is 'PHP is cool'
View Source Code