how php and mysql work together?

my apache works well and php works well too,

then I install mysql, it also works, now what I need to do is let php and mysql work together, My server is on running, so I have to be carefull, must not cause server down,may someone here would like to tell me the easy and safe way?thank in advance.

Go to http://dcfonline.sfu.ca/ying/linux/

and click on Web Database Step-by-Step Guide

It has a very basic but clear explanation of how it works.