What are some useful algorithms?

What are some useful algorithms you find yourself re-using in lots of your work?

I ask this because I would like to practice my php skills, and I figure if I am going to practice making some scripts I might as well make them useful for me in the future.

Thanks!

You could spend some time writing bubble sort algorithms, or if you want something more useful you could write a string tokenizer. Writing parsers is great fun :)

Perhaps you'd also be interested in design patterns; I like the MVC and ORM patterns, personally.