On the BinaryTides blog there's a series of posts that share some tips and suggestions aimed at helping you and your code be the best they can be - things to enhance your application (including suggestions not just about code but also about environment and development practices).
- Maintain debugging environment in your application
- Collect all output at one place , and output at one shot to the browser
- Set the correct character encoding for a mysql connection
- Do not gzip output in your application , make apache do that
- Don't check submit button value to check form submission
- Process arrays quickly with array_map
- Avoid direct SQL query , abstract it
- Never set error_reporting to 0
- Make a portable function for executing shell commands
Obviously, not all of these will apply in all situations, but they're an interesting list. Most will come with good explanations and code samples when appropriate.