What are common pitfalls when trying to access functions from one class in another class in PHP?

One common pitfall when trying to access functions from one class in another class in PHP is not properly including the file that defines the class. To solve this, make sure to include the file containing the class definition before trying to access its functions in another class.

// Include the file containing the class definition
include 'class1.php';

// Create an instance of the first class
$class1 = new Class1();

// Access a function from the first class in the second class
echo $class1->functionName();