what's the difference between php4 and php5?

Answer 1

look out for php7!!!

Answer 2

i think PHP5 is upgraded version of PHP4.
So PHP5 having some new features. Some of them are as following:
1) In PHP5, you have to name your constructors and interfaces are introduced.
2) In PHP5, Class can be declared as Abstract and introduced a special function which is __autoload(). In PHP5, Class or method can be declared as Final.
3) In PHP5, Magic methods are introduced such as __call, __get, __set and __toString.
4) In PHP5, 'exceptions'(exception errors) has introduced.