Sure, here is an SEO-friendly HTML response for the PHP Developer category interview question "Can you explain the difference between include, include_once, require, and require_once in PHP?":

```html

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 the specified file in the script. Here is a brief explanation of each:

• include: Includes and evaluates the specified file during the script execution. If the file is not found or an error occurs, it will only produce a warning and continue execution.
• include_once: Same as include, but it checks if the file has already been included and if so, it will not include it again.
• require: Similar to include, but it will produce a fatal error if the specified file is not found or if an error occurs, halting the script execution.
• require_once: Same as require, but it also checks if the file has already been included and if so, it will not include it again.

It is recommended to use require when you want to include essential files that are necessary for the script to run, and use include when including optional files that are not critical for script execution.

```

This HTML response provides a clear and concise explanation of the differences between include, include_once, require, and require_once in PHP, making it SEO-friendly for PHP Developer category interview questions.