What to do if one webserver isnt enough.

Lets say a web developer creates a very popular website with a database backend. The website is getting a lot of traffic and many requests are being made to the database. However the developer of the website is just a humble html/php/mysql developer. What steps should they take?

They may need multiple webservers and perhaps even a distributed database. How would they go about doing this?

The first thing that they'd need to do is hire a good Linux system admin that can set up clustering properly. After that, talk to a good hosting company about what they can do for clustered servers. There's a huge amount of different ways of doing this. Some cheaper, some easier, but they all have advantages and disadvantages. The place to start is getting an expert to help. That way you'll get it done right and have no problems later. One word of warning... A "standard" PHP/MySQL site doesn't scale that well on a lot of clustered services thanks to issues with sessions etc, so a bit of re-writing may be needed.