Length of the string 'Hello World!' is: 12
Hello, Amr!
Explanation of Functions:
Functions in PHP are blocks of reusable code that perform specific tasks. They can be built-in functions provided by PHP or user-defined functions created by the programmer. Functions help in organizing code, improving readability, and promoting reusability.
A built-in function, like `strlen()`, is a function that is already provided by PHP and can be used directly. A user-defined function, like `greet()`, is a function created by the programmer to perform a specific task.