There are two ways of doing PHP:
- the old way (basically php3/php4) where no classes are used and everything is basically in functions and you just call them when you need them
- the new way (OOP in php5/php6) and it's mostly used in combination with various MVC frameworks (CodeIgniter, Yii, phpFuel, Cake, Angular) and this is very useful when: you will need to reuse the part of the code, when several people work on same project, when someone doesn't have to read all your functions before it can start some coding etc...
Don't make me wrong, classes in php aren't new, but lately their use in projects increased as language developed and people wanted to fully move to OOP.
Here's one useful php link:
PHP: The Right Way