WordPress has an extended API which is documented in Developer Resources. This comes extremely handy when you are developing your new custom addon. Even though it is absolutely correct to use pure PHP for your implementation, there are several core functions you can use and save time.
For example if you wish to load your own custom CSS style file, you can utilise the wp_enqueue_style() core function and specify the proper parameters for this file.
Furthermore, even for those parameters, there are already implemented core functions you can use. The second parameter for the src of this file can be a URL, or the path relative to the WordPress root directory. Although this is absolutely correct to specify this location as a string, you can alternatively get the active theme URI, the parent theme URI or the URI of a file in the theme folder using one of the following core functions respectively:
In general, core functions and the API save you from a lot of trouble and time when developing for WordPress, like the user on the following topic,
and is the preferred way to go for when developing your new, exciting idea on WordPress.