First step would be to duplicate your content in Hindi(flag it differently in the database(or file system, different directories for each language) ie lang='en' for english, lang='hi' for hindi).
Then, you have 2 options: Use JS to detect the users language settings, and display the correct content based on that:
JavaScript for detecting browser language preference
Or, you can just create a drop down option(or any other type of selector) somewhere on your site that allows them to select their preferred language, and again serve the correct content based on that option.
EDIT*
If you want to do it purely with PHP, you'd have to detect the language setting in the User-Agent string.
Detect Browser Language in PHP