What is the difference between include and require in PHP?

The difference between include and require in PHP primarily revolves around how they handle errors. When using include, if the specified file cannot be found, PHP will emit a warning, which means that the script will continue to execute despite the missing file. This is important in scenarios where the file is not crucial for the execution of the rest of the script, allowing for a level of flexibility.

In contrast, when require is invoked and the specified file cannot be included, PHP generates a fatal error, immediately halting the execution of the script. This is significant for cases where the functionality provided by the included file is essential for the rest of the script to operate correctly. Therefore, using require ensures that a script does not proceed in a potentially unstable state due to a missing critical file.

This distinction highlights why the identification of include generating a warning and require resulting in a fatal error is critical in PHP programming, as it allows developers to choose between a soft failure and a hard stop depending on the application's requirements.