The uwawidget.php library allows a facilated use of UWA-Widgets with PHP. It provides access to the preferences and some convenience-functions. You can test a limited amount of Widgets here, but in theory, it should work with every UWA-compliant Widget.
The PHP UWA Library can be downloaded from the oncode.info main site.
You can find widgets on the Netvibes Widget Directory. Look at the Source link in the widgets details. Take care, just UWA-Widgets work (orange icon), Mini-APIs sometimes do.