What is the error message "Fatal error: Call to undefined function" indicating in PHP?

The error message "Fatal error: Call to undefined function" in PHP indicates that the code is trying to call a function that has not been defined or loaded. This can happen if the function is misspelled, not included in the code, or from a different file that is not being included. To solve this issue, make sure the function is properly defined and 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();