New syntax is added to the language to simplify the writing of the program and simplify the organization of the code; usually don't do it better. You should not think about using specific syntax for performance reasons, but because it makes your code better, easier to work, easier to read, etc.
For example, PHP 5.5 introduces the Generator syntax ( yield keyword). This is an improved way to write iterators. But it will not provide a performance difference between writing Iterator style code; it will basically do the same under the hood. The important point is that it is much easier to write a simple generator than to write the same code using Iterator. The code is shorter, neater and more understandable. He will not work faster, but it will be much easier to work with him. It's all.
However, PHP does have a good track record that significantly improves language performance with each version. You don’t even need to use the new features - just upgrade to the latest version and you will get a performance boost from your existing code. In some cases, productivity can improve by 50% or more. Considering that the update should be no problem; the ability to use new language features is an added advantage over performance gains.
There are many criteria to prove this; here is one example that covers PHP v5.1 to 5.4.
(I have not seen a good test for v5.5 yet, apparently because it still works, but you can be sure that the tests will be published shortly after its release)