How can I use php to categorise different keywords together for example to consider shoes, boots, nike, etc in the similar categories.
Any code would be appreciated.
(Warning: a fundamentalist approach) Look at MIT Reality Commons [0] and OpenCyc [1], [2]. These are two open databases of common sense. Make several searches by categories you're interested in. You'll get some related terms for each category. Put it in a fast database of your liking, and you're set.
Also, various SEO people like to create clouds of related keywords in meta tags of relevant pages. Take a look at source of several such pages, extract and filter keywords.