What are the different include commands in PHP and when should each be used?

There are three different include commands in PHP: include, require, and require_once. - include: This command includes and evaluates the specified file during the execution of the script. If the file is not found, a warning is issued, but the script continues to execute. - require: This command is similar to include, but if the file is not found, a fatal error is issued, and the script stops executing. - require_once: This command is similar to require, but it ensures that the specified file is included only once in the script, even if it is called multiple times. Here is an example of using the include command:

```php
<?php
include 'header.php';
?>
```

Here is an example of using the require command:

```php
<?php
require 'config.php';
?>
```

Here is an example of using the require_once command:

```php
<?php
require_once 'functions.php';
?>