The document discusses PHP functions, including how to create and call custom functions, and examples of useful built-in PHP functions. It explains that functions allow reusable blocks of code and built-in functions are pre-made and do not need to be created. Examples are provided for creating functions that take parameters and return values, as well as calling functions. Common built-in functions are also described, such as trim(), explode(), implode(), and print_r() which operate on strings and arrays.