I found PHPBench.com when i was stumbling. It is a simple but good site that lists some performance issues with basic php functions and usage of them.
PHPBench.com was constructed as a way to open people’s eyes to the fact that not every PHP code snippet will run at the same speed. You may be surprised at the results that this page generates, but that is ok. This page was also created so that you would be able to find discovery in these statistics and then maybe re-run these tests in your own server environment to play around with this idea yourself, by using the code examples (these code examples are automatically generated and as the code in my .php files change, so do they).
You should take a look to see basic differences of using those functions and what you are doing right / wrong.