What is the difference between include(), require(), include_once(), and require_once() in PHP, and when would you use one over the other?

Difference Between include(), require(), include_once(), and require_once() in PHP

include(): This function includes and evaluates a specific file during the execution of the script. If the file is not found, a warning is issued but the script will continue to execute.

require(): Similar to include(), but require() issues a fatal error if the file is not found, halting the script execution.

include_once(): This function includes a file only once during a script's execution, preventing multiple inclusions of the same file.

require_once(): Similar to include_once(), but it will issue a fatal error if the file is not found, and ensures that the file is included only once.

When to Use Each Function:

Use include() or require() when you are including a file that is essential for the script to run properly. Use require() if the file is crucial, and you want to halt the script execution if it is not found.

Use include_once() or require_once() when you need to include a file that might already have been included in the script to prevent duplication.

When in doubt, it is generally recommended to use require() or require_once() to ensure that essential files are properly included in the script.