php if statement relating to the url address

[velo]

Say my URL address is www.example.comI want to have an if statement in my script such that the test condition is true if the url is www.example.com and false if it is anything else e.g. www.example.com/test or www.example.com?var=3&function=7.How do I achieve this? And will it add much overhead to the loading of the webpage? As I will have this if statement on the front page of the website. Thank you