What is the difference between PHP require and include?

I know the basic usage of PHP require, require once, include and include once. But I am confused about when I should use them.

Example: I have 3 files, eg: settings.php, database.php, and index.php.

In database.php file, i wrote:

require_once 'settings.php';

and then in index.php, i wrote:

require_once 'settings.php';
require_once 'database.php';

so I load settings.php two times, is this okay? Any tips using these functions?

• include includes a file and throws a warning if the file was not found.

• require includes a file and throws a fatal error if the file was not found.

• include_once and require_once do the same thing, but only if the file was not already loaded.

However, the need for one of the _once functions is usually a sign of bad design. You should build your scripts in a way that clearly defines what gets included where.

Choose one place for settings.php to get included - probably index.php. There should be no need to additionally include it in database.php.

You don't load settings.php two times, as per PHP documentation on require_once;

The require_once() statement is identical to require() except PHP will check if the file has already been included, and if so, not include (require) it again.

The difference between include() and require() arises when the file being included cannot be found: include() will raise a warning (E_WARNING) and the script will continue, whereas require() will raise a fatal error (E_COMPILE_ERROR) and halt the script. If the file being included is critical to the rest of the script running correctly then you need to use require().

For more details : Difference between Include and Require in PHP