What are some common mistakes beginners make when working with PHP classes and functions?

One common mistake beginners make when working with PHP classes and functions is not properly including files that contain the class definitions or function declarations. This can lead to errors such as "Class not found" or "Undefined function" when trying to use the class or function. To solve this issue, make sure to use the `require` or `include` functions to include the necessary files before trying to use the classes or functions.

// Incorrect way of including file
include 'my_class.php';

// Correct way of including file
require 'my_class.php';