This Phing filter takes a fileset of CSS files and looks for urls and adds timestamps reflecting the modification time of the resource.
Here is an example target
<target name="version-css-urls">
<reflexive>
<fileset dir="css">
<include name="**/*.css"/>
</fileset>
<filterchain>
<filterreader classname="VersionCSSURLs"/>
</filterchain>
</reflexive>
</target>