On his blog today Arun Gandhi has posted an "all in one place" list of technology that you can use to help speed up your site overall without too much effort on your part.
PHP is great for writing quick dynamic stuff for your website. Just a couple of lines of code can be written in 2 mins to insert or retrieve data from db. But with ease there is also some pain… the downside is that each request for a dynamic page can trigger multiple db queries, processing of output, and finally formatting to display on browser. This process can eventually be slow on larger sites or slower servers.
There's lots of options here for all different levels of the code - opcode caching to speed up repetitive operations, data caching at the PHP layer, caching plugins and template caching.