PHP3 is an extension of files specific to the scripting and interpreted PHP programming language. PHP3 is a file used to store program code written in that language. It is a technology whose main application is the creation of websites and web applications.

The code contained in PHP3 files is executed on the web server and on its basis an appropriate HTML file is generated and sent to the user's browser. The PHP3 extension means that the code contained in the file is compatible with PHP version 3.0.