Original string: Hello, world!

Uppercase: HELLO, WORLD!

Hello, Eman!

Functions in PHP are blocks of reusable code that perform a specific task. They can take input parameters, process them, and optionally return a result.

Built-in functions, like strtoupper(), are provided by PHP and can be used directly. User-defined functions, like greet(), are created by the programmer to encapsulate specific tasks.

Functions improve code organization, readability, and reusability by allowing you to define a task once and call it multiple times throughout your code.