You might wonder why anyone would want to use MySQL functions when PHP comes with a whole bunch of powerful functions of its own. The answer is very simple: the MySQL functions work on the data right there in the database. If you were to use PHP, you would first have to extract raw data from MySQL, manipulate it, and then perform the database query you first wanted.
By having functions built into MySQL, the time needed for performing complex queries is substantially reduced, as is their complexity. If you wish to learn more about the available functions, you can visit the following URLs:
String functions: http://dev.mysql.com/doc/refman/5.0/en/string-functions.html
However, to get you started, Appendix D describes a subset containing the most useful of these functions.