Built-in Function <br><br>The phrase "I love to count!" contains 16 characters.<br><br><br>User-defined Function <br><br>2 + 4 = 6<br>12 + 33 = 45<br>1 + 3 = 4<br><br>A function is a reusable code that performs a specific task. 
Predefined functions, like strlen(), are already included in PHP and can 
be used right away. User defined functions are created by the coder <br>
using the function keyword, and can accept input through parameters and 
return a result using the return keyword. Functions help organize code, 
avoid repetition, and make programs easier to read and maintain.