For example if we consider ActionScript2.0(based on Objects but programming does not implement much OOP ) vs 3.0(highly OOP) its like a whole new scripting language in the sense of approach, programming style,features you get the idea.

In PHP we can see current versions going from 3-5. brief version changes

• Question :Developers who work on PHP is it easy to migrate from version to version?
• Question :Are there any extensive compatibility issues, forward or backward?
• Question :Does your project stick to a particular version till the end ?
• Question :Does the programming style ,approach change from version to version?
• Question :If you had to get started on PHP to contribute to a project built earlier versions, would learning the latest version be counterproductive towards this aim?

Some related topics i had come across on SE

How should I be keeping track of php script version/changes?

What is happening to PHP 6?

It would be Really helpful in understanding if you could answer this topic directly to the questions put forth.