For questions about the server-side programming language that the WordPress core, plugins and themes are written in. Questions about using PHP outside a WordPress context are off-topic but might be asked on Stack Overflow.
PHP stands for PHP: Hypertext Preprocessor, and it's one of the most widely used languages for developing web applications. It is an interpreted and weakly typed language that shares much of its syntax, conventions and function names with C. It is developed as an open source project.
General PHP questions should be asked on Stack Overflow, but questions specific to using PHP in a WordPress context may be suitable for WordPress Development.