Doug Wollison

This plugin is no longer being developed.

QuickStart

This plugin is no longer being developed.

Steven Vachon

If you were to use this on a website like http;//example.com/test/testing/, you would get results like these:
<ol>
<li>
<ul>
<li>Before: http;//example.com/test/another-test/#anchor</li>
<li>After: ../another-test/#anchor</li>
</ul>
</li>
<li>
<ul>
<li>Before: http;//example.com/wp-content/themes/twentyten/style.css</li>
<li>After: /wp-content/themes/twentyten/style.css</li>
</ul>
</li>
<li>
<ul>
<li>Before: https;//example.com/wp-content/themes/twentyten/style.css</li>
<li>After: https;//example.com/wp-content/themes/twentyten/style.css</li>
</ul>
</li>
<li>
<ul>
<li>Before: http;//google.com/test/</li>
<li>After: //google.com/test/</li>
</ul>
</li>
<li>
<ul>
<li>Before: ../../../../../../../../#anchor</li>
<li>After: /#anchor</li>
<li>After (<code>$output_type=1</code>): ../../#anchor</li>
</ul>
</li>
</ol>
All string parsing. No directory browsing.
If you’re looking for a plugin that will automatically convert all URLs on your WordPress site, instead check out my other plugin, <a href="https://wordpress.org/extend/plugins/wp-html-compression/" rel="ugc">WP-HTML-Compression</a>. 
Before you copy this code and add it into your own, keep in mind that there will probably be future updates. Keeping the code within an installed plugin will make sure that you’re notified.

Absolute-to-Relative URLs

A function() for use in shortening URL links. This plugin is meant for dev work and does not automatically shorten URLs.

QuickStart vs Absolute-to-Relative URLs