What are the differences between include, require, and require_once functions in PHP?

The main differences between include, require, and require_once functions in PHP are: 1. include: Includes and evaluates a specified file during the execution of the script. If the file is not found, a warning is issued, but the script continues to execute. 2. require: Includes and evaluates a specified file during the execution of the script. If the file is not found, a fatal error is issued, and the script stops executing. 3. require_once: Same as require, but ensures that the specified file is included only once to avoid duplicate function/class declarations.

// Example of using require_once to include a file
require_once 'myfile.php';