index.php is the only file that Joomla insists is present in a template.

It is where all the main logic of a template is stored and has to specify all the module and component display code. Although any JavaScript code would also be referenced here, it is probably a bad idea to include formatting and layout directives in this file.

Basic Functionality

A minimal Joomla index.php would look something like:

  <html>
  <head>
     <jdoc:include type="head" />
  </head>
  
  <body>
     <h1> This is a Joomla heading entry</h1>
  </body>
  </html>

Although Joomla will run this satisfactorily, it will actually produce no visible output in the browser. So, what is actually happening?

JDocument Class

The “jdoc” tag sets up a call to JDocument, which is a PHP class within the Joomla framework. The modifier ”:include” specifies a method within this class. Generally speaking, the include method is used for all output within Joomla templates.

JDocumentHTML Class

This provides access to the generated document text in a structured fashion. The most useful properties of this object are…

direction specifies the reading direction of output text.
template holds a path string that tells us where the template is stored.
title is the document title used for display, etc.
description a reprise of the HTML metadata describing the generated document.
link holds the base URL of the generated document.
language specifies the generated document's language code e.g. “en” for “English”.

JDocumentHTML methods include:

countmodules returns the number of modules active on the current page. If given the name of a module as a parameter, it will instead return the number of instances of that particular module, which are active on the page.

Using $this

Within a Joomla template, $this always returns a reference to a current instance of JDocumentHTML. Hence the statement

 <?php echo $this->template;?>

will print the current template's directory path.