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URL-Based Session IDs

URL-based session management causes major problems for search engines, because each time a search

engine spiders your web site, it will receive a different session ID and hence a new set of URLs with the

same content. Needless to say, this creates an enormous amount of duplicate content. The PHP feature

that automatically tracks user sessions using a query string parameter is named trans_sid. You can

disable this feature, and permit only cookie-based session support.

To turn off URL-based session IDs, you’d need to add these lines to your

.htaccess

file:

php_value session.use_only_cookies 1

php_value session.use_trans_sid 0

The same effect can be achieved using this PHP code:

<?php

// store the session ID using cookies

@ini_set (‘session.use_only_cookies’, 1);

// disable trans_sid

@ini_set (‘session.use_trans_sid’, 0);

?>

The URL factory you created in Chapter 3, together with the redirect library from Chapter 4, can be used

to redirect any URLs that contain the session ID to the “proper” versions of the URLs in case this feature

was inadvertently left on and such URLs are indexed by a search engine.

Chapter 11 also discusses a method using cloaking that dynamically turns URL-based session IDs off for

search engines, but leaves it on for human users.

Other Navigational Link Parameters

In general, parameters in URLs such as those that indicate that a user came from a particular page can

create a large amount of duplicate content. Covering all examples would be impossible, but consider the

following imaginary URLs:

http://www.example.com/Some-Product.html?from_url=about-us.php

http://www.example.com/Some-Product.html?from_url=contact-us.php

The list could get quite long depending on how many pages link to that product. In practice, whenever

possible, it may be advisable to use session-based or HTTP_REFERER-based data to track things such as

these, even if imperfect solutions.

URL-based sessions may be important on large e-commerce sites, or in certain demo-

graphics, because a small number of users disable cookies in their browsers.

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Chapter 5: Duplicate Content

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