Include external files for php, javascript, and css

We can include external files for PHP, javascript, and css. Using external files is really good for us because we can save a lot of time coding by using it more and more time.
For example, you have already make a css file to custom your html code for all your web page, then you can just link it to every page on your html file. So if you want to change the color of your web page background, you just need to change only 1 file and it will affected to all your web page. Here is the way to include external file.
For php:
include ("filename.php");
require ("filename.php");
include and require are same but they are different when there is an error occurs.
include will generate warning but it continues the script. require will generate fatal error and stop the script.

For javascript:
<script type="text/javascript" src="filename.js"></script>

For css:
<link rel="stylesheet" type="text/css" href="filename.css" />

Hope this can help you