Explain the difference between include(), require(), include_once(), and require_once() in PHP, and when would you use each one?

Explanation of include(), require(), include_once(), and require_once() in PHP

include(), require(), include_once(), and require_once() are all PHP functions used for including external files in a PHP script. Here are the differences between them:

1. include(): This 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 to execute.
2. require(): Similar to include(), require() includes and evaluates a specified file during the execution of the script. The difference is that if the file is not found, a fatal error is issued and the script stops executing.
3. include_once(): This function includes and evaluates the specified file only once during the execution of the script, even if it is called multiple times in the script.
4. require_once(): Similar to include_once(), require_once() includes and evaluates the specified file only once during the execution of the script. If the file is not found, a fatal error is issued and the script stops executing.

When to use each function:

• Use include() or require() when you want to include a file that may not be crucial for the script to continue execution.
• Use require() when you want to include a file that is essential for the script to function properly.
• Use include_once() or require_once() when you want to ensure that a file is included only once, to prevent redeclaration errors.