Differences between include(), require(), include_once(), and require_once() functions in PHP

The include() function 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 running.

The require() function is similar to include(), but if the specified file is not found, a fatal error is issued and the script execution halts.

The include_once() function includes a file only once in a script. If the file has already been included, it will not be included again.

The require_once() function works the same as include_once(), but with the difference that it will issue a fatal error if the file is not found.