Optimising your server side scripts such as ASP and PHP is very important for better utilisation of server resources. Even a high end server can fail to deliver the speed and performance that you look for, if the codes are not optimised.
For PHP & My SQL:
 It is a good method to store connection string in a common file. This common file can be included as a header in all the pages, wherever it is required.
 IMPORTANT : Avoid using multiple loops in a single pages. Most of the programmers make mistakes in handling loops thus the asp pages will not be executed after few hits.
 Do not unnecessarily use asp pages. It is strongly adviced to keep the home page as ".html" and not as ".php"
 Table Size should be kept at minimum. Avoid opening multiple tables at a time. Moreover, avoid querying a single large table.