A common use of include is to separate page-specific content from general site design. Common elements such as headers and footers go in separate HTML files, and each page then looks like:

<? include 'header.html'; ?>
content
<? include 'footer.html'; ?>

We use include because it allows PHP to continue to process the page even if there's an error in the site design file(s). The require construct is less forgiving and is more suited to loading code libraries, where the page can't be displayed if the libraries don't load. For example:

require 'codelib.inc';
mysub( );               // defined in codelib.inc

A marginally more efficient way to handle headers and footers is to load a single file and then call functions to generate the standardized site elements:

<? require 'design.inc';
   header( );
?>
content
<? footer( ); ?>

If PHP cannot parse some part of a file included by include or require, a warning is printed and execution continues. You can silence the warning by prepending the call with the silence operator; for example, @include.

If the allow_url_fopen option is enabled through PHP's configuration file, php.ini, you can include files from a remote site by providing a URL instead of a simple local path:

include 'http://www.example.com/codelib.inc';

If the filename begins with "http://" or "ftp://", the file is retrieved from a remote site and then loaded.

Files included with include and require can be arbitrarily named. Common extensions are .php, .inc, and .html. Note that remotelyfetching a file that ends in .php from a web server that has PHP enabled fetches the output of that PHP script. For this reason, we recommend you use .inc for library files that primarily contain code and .html for library files that primarily contain HTML.

Code in an included file is imported at the scope that is in effect where the include statement is found, so the included code can see and alter your code's variables. This can be useful—for instance, a user-tracking library might store the current user's name in the global $user variable:

// main page
include 'userprefs.inc';
echo "Hello, $user.";

The ability of libraries to see and change your variables can also be a problem. You have to know every global variable used by a library to ensure that you don't accidentally try to use one of them for your own purposes, thereby overwriting the library's value and disrupting how it works.

If the include or require construct is in a function, the variables in the included file become function-scope variables for that function.

for ($i=0; $i < 10; $i++) {
  include "repeated_element.html";
}