When writing new code it’s really helpful if developers adhere to a few basic rules and conventions. PHP is a fairly lose language which lets developers do things in a multitude of different ways, while this gives a creater level of flexibility and makes picking up PHP quick and easy (compared with some other languages), it also means that code can easily become confusing for others to pickup; with one developer effectively not understanding the ‘hand-writing’ of another.
So having a pre-determined set of coding conventions makes a lot of sense. Much of the work we do at 18a these days is based on the CodeIgniter framework, so whenever we can, we adhere to their style guide.