The callback function
Callback function: Passes a user-defined “execution” argument to a function, greatly increasing the number of callback extensions to the function’s functionality: A PHP callback is a function that is passed as an argument. A PHP callback is a function that suddenly jumps from the main thread and returns to the main thread to process the rest of the process.
Recursive function
Recursive function: a function calls itself, but it must be judged before calling itself, otherwise the call will continue indefinitely.
Three, arrays,
1. Array definition
Array: A collection of ordered members (key-value pairs). 1. Indexed array 2. Associative array
2. Array functions
- Range (): Creates an array quickly