A function is a block of code that implements a specific function. The direct purpose of defining a function is to divide the program into blocks according to functions, so as to facilitate the use, management, reading and debugging of the program. Functions can be roughly divided into system functions and custom functions. When the functions provided to us in the system cannot achieve a specific function you want to achieve, you need to define a function yourself. This is called a custom function. , of course there are some special forms of functions in PHP, such as recursive functions, callback functions, etc.