Difference between php4 and php5

1. Unified constructor and Destructor.
2. Exception has been introduced.
3. New error level named E_STRICT has been introduced.
4. Now we can define full method definitions for a abstract class.
5. Within a class we can define class constants.
6. We can use the final keyword to indicate that a method cannot be overridden by a child.
7. Public, private and protected method introduced.
8. Magic methods has been included.
9. We can pass value by reference.