We will make a flowchart of the PHP script that will be able to place the canonical tag at the source of any non-canonical URL issues without the use of .htaccess or redirections.

Basically there are four domain canonical scenarios:

• A domain using www and using the non-secure protocol (http://) as the canonical version. Most of the domains on the Internet fit into this scenario.
  
• A domain using www and the secure protocol (https://) as the canonical version. A good example of this is PayPal.
  
•  A domain using non-www and using the non-secure protocol (http://) as the canonical version. This makes for easier typing of domain names in the browser for faster user access. A good example of this is Digg.
  
• A domain using non-www and using the secure protocol as their canonical version. This alternative is not popular, although some good sites use this type of canonical version solution, such as https://launchpad.net/

Each of these URL canonical scenarios requires different PHP scripts, but the flow of implementation is the same. Here is an example that illustrates a site using http:// and www: as their canonical version.

Step 1: If the domain is using session IDs, this should be stripped off first as it is not needed as part of the canonical version.

Step 2: Check to see if the requested protocol is http://. If it is not, return the http version to the browser and append results with Step 3.

Step 3: If the requested URL dos not use www, append www to the URL; otherwise, return it to the browser.

The final canonical URL should:

• Not use a session ID.
  
• Contain the www version. 
  
• Use the canonical protocol, which is http://

In the rest of this article, I'll show you the recommended PHP canonical scripts for the four scenarios discussed above.