What are the differences between using readfile and include in PHP, and when should each be used?

The main difference between using readfile and include in PHP is that readfile simply reads a file and outputs its contents, while include actually executes the PHP code within the included file. Readfile is typically used for serving files like images or PDFs, while include is used to include PHP code from another file into the current file. If you want to include a PHP file and execute its code, use include. If you simply want to output the contents of a file, use readfile.

// Using include to execute PHP code from another file
include 'example.php';

// Using readfile to output the contents of a file
readfile('example.txt');