It sounds to me like you were doing feature testing, and maybe some unit testing.
Unit testing would be putting print statements into your php code to show it's working properly, or checking that you have done used a mysql api correctly. Basically that you don't have any obvious coding errors. If you have instrumented your code so you can "turn on" and "turn off" debug messages ("insert successful", "lookup found, name is: ", $name) etc then you will have a good idea if your code is working properly.
feature testing is checking out the functionality of the site from a user's view. Does it support the functions it should (inserting / deleting / edit - database management stuff) and are they implemented correctly. If you take the time to write down what the system should do from the user's viewpoint, you will get a sense of how well you've done feature testing.
What's left is system testing. For a php website like this, you would want to have an idea of how many accesses you expect to the site, what people will be doing. If you have enough equipment, you would then create a load on your site and see how it behaves under stress (crank that load up as far as it can go and see when the system breaks down), reliability, performance (transaction time...) etc. Systems houses use tools like loadrunner etc to do this. But you can also find a huge world of tools at:
http://opensourcetesting.org/performance.php
And other info here:
http://www.qadownloads.com/White_Papers/
If you are hosting the server on your own linux installation you will want to subect the server to the kinds of loads you expect it will see from user traffic.
Good Luck!