How can one prevent the error "Call to undefined function = Aufruf einer unbekannten Funktion" in PHP?

The error "Call to undefined function" occurs when a function is called in PHP that has not been defined or loaded. To prevent this error, ensure that the function is defined or included in the code before calling it.

// Define the function before calling it
function myFunction() {
    // Function code here
}

// Call the function after defining it
myFunction();