The first version of PHP was just a way to embedd server side tags in pages for things like hit counters.
People wanted more and more features added, till somebody eventually added an IF tag and the race was off to create a real languages.
As PHP was used for more and more complex projects, the capacity for OO programming was more desirable.
OO is overkill for a lot of trivial web projects. But if you need to create a truely complex and powerful system, it is a big advantage.
There is a reason PHP is on version 5 when most other languages only have two or three versions no matter how old they are.
Jay