Don't use a regex if you don't have to

PHP has a rich set of string manipulation functions - use them!

References?

It is usually not faster to use references because of PHP's copy-on-write nature.

Use Persistent Database connections

Some database are slower than others at establising new connections. The slower it is, the more of an impact using persistent connections will have. But, keep in mind that persistent connections will sit and tie up resources even when not in use. Watch your resource limits as well. For example, by default Apache's

Using MySQL? Check out mysql_unbuffered_query()

Use it exactly like you would mysql_query(). The difference is that instead of waiting for the entire query to finish and storing the result in the client API, an unbuffered query makes results available to you as soon as possible and they are not allocated in the client API. You potentially get access to your data quicker, use a lot less memory, but you can't use mysql_num_rows() on the result resource and it is likely to be slightly slower for small selects.

Hey Einstein!

Don't over-architect things. If your solution seems complex to you, there is probably a simpler and more obvious approach. Take a break from the computer and go out into the big (amazingly realistic) room and think about something else for a bit.