Only use rÃÃferences where it is really needed, references do not increase performance (as one might think). There are many pitfalls with reference usage so stay away.

Don't use @ in front of functions, this makes it much harder to debug. If you have to, a comment before it is required.

Use real static functions, never use fake static (or semi-static) functions. Real static functions will not include the $this variable from the calling function.

Avoid dependencies at all costs, each class should be a small a unit as possible.

Use join() function instead of implode(), this makes the code easier to read.