The Difference Between include, include_once, require, and require_once in PHP

include, include_once, require, and require_once are all PHP functions used to include and evaluate external PHP files in your code. The main difference between them is how they handle errors and file inclusions.

include

The include function includes and evaluates the specified file during the execution of the script. If the file is not found or included multiple times, it will produce a warning but the script will continue running.

include_once

include_once is similar to include but it will only include the file once. If the file is already included, it will not be included again. This can help prevent duplicate function redefinitions and variable assignments.

require

The require function is similar to include but it will produce a fatal error and stop the script if the specified file is not found. require is used when the included file is essential for the script to function correctly.

require_once

require_once is similar to require but, like include_once, it will only include the file once. If the file is already included, it will not be included again. This can help prevent fatal errors when multiple files are included.

When to Use Each Function

• Use include when the included file is not critical for the script to run and you want to continue script execution even if the file is not found.
• Use include_once when you want to prevent duplicate file inclusions and potential issues with redefined functions and variables.
• Use require when the included file is essential for the script to function correctly and you want to halt script execution if the file is not found.
• Use require_once when you want to prevent fatal errors from multiple inclusions of the same file.