The length of the string 'Hello, World!' is 13.
The strlen() function is a built-in PHP function that returns the length of a string.
Hello, Alice!
The greet() function is a user-defined function that takes a name as an argument and returns a greeting message.
Functions in PHP are blocks of code that can be reused. Built-in functions are provided by PHP, while user-defined functions are created by the programmer. Functions help to organize code, make it more readable, and reduce repetition by encapsulating reusable logic.