Can you explain the difference between include(), require(), include_once(), and require_once() functions in PHP and provide examples of when you would use each one?

Understanding include(), require(), include_once(), and require_once() functions in PHP

When it comes to including external files in PHP, there are four main functions that are commonly used: include(), require(), include_once(), and require_once(). Each of these functions serves a specific purpose and has its own usage scenarios.

include() Function:

The include() function includes and evaluates the specified file. If the file is not found or is unable to be included, a warning is issued, but the script execution continues.

Example:


```php

```


require() Function:

The require() function is similar to include(), but it generates a fatal error if the file cannot be found or included, halting the script execution.

Example:


```php

```


include_once() Function:

The include_once() function includes the specified file just once. If the same file has been included previously, it will not be included again.

Example:


```php

```


require_once() Function:

The require_once() function works similarly to include_once() but generates a fatal error if the file cannot be included or found, ensuring that it is included just once.

Example:


```php

```


Understanding the differences and appropriate use cases of include(), require(), include_once(), and require_once() functions in PHP can help in structuring your code effectively while handling file inclusions.