In PHP, User-defined functions are created by programmers to meet specific requirements, while PHP built-in functions are provided by PHP to perform common tasks without the need for manual implementation. Both types of functions play crucial roles in PHP development, offering flexibility, modularity, and efficiency in coding.

User-defined Functions:

• User-defined functions are created by the programmer to perform specific tasks according to their requirements.
• These functions are declared using the function keyword followed by a function name and a block of code encapsulated within curly braces {}.
• They can accept parameters (input data) and return values (output data) using the return statement.
• User-defined functions provide a way to encapsulate reusable code, promote modularity, and enhance code readability and maintainability.

Syntax:

function greet($name) {
    return "Hello, $name!";
}

Built-in Functions:

• Built-in functions, also called predefined or library functions, are included in PHP by default.
• They perform common tasks like handling strings, arrays, files, math operations, and databases.
• These functions save time and effort as they are already implemented and available for use.
• PHP offers a vast range of built-in functions, providing developers with tools to complete tasks quickly and efficiently.

Syntax:

$string = "Hello, world!";
$length = strlen($string); 

// Built-in function to find the length of a string

The table below illustrates the user-defined function and built-in function in PHP along with corresponding descriptions.