Normally, if you want to run PHP scripts on a web page you will give that page a .php file extension. Otherwise, the PHP script will not be parsed (or executed). However, if the page already exists with a .htm or .html file extension, it may not be convenient or desirable to change the extension.
Fortunately, there is a simple alternative to changing the file extension. You can instead add a simple instruction to a .htaccess file, as this post demonstrates.
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An external style sheet allows all of a site’s styles to be controlled from one point—which makes for very efficient page layout and design.
“PHP includes” can be used in a similar way to external style sheets, in that elements appearing on multiple pages can be controlled from one location. For example, it is common to have a menu on every page of a website. If you want to update that menu—such as adding a new menu item—it will be a nightmare to do this if you have to open every page on your site to alter each instance of that menu.
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