I just finished writing my article for the next issue of PHP Magazine, and as promised, I go into greater detail about optimizing, staticizing, and caching PHP to deliver high performance in heavy traffic. In the section on optimizing source code, I cite the three major areas where I have most commonly seen bottlenecks:
dealing with large objects
dealing with network operations
dealing with database queries
The article goes into detail on how to deal with these three areas effectively to reduce memory overhead and execution time, making your web application run nice and snappy.