Are sites with the .php extension worth linking with?

Will they be spidered if they do not have an html extension?

Check to see if the page is indexed by the search engine. If it is, then absolutely, it is an effective link. Google in particular indexes Word files (.doc) and Acrobat files (.pdf) in addtion to HTML. Moreover, php, asp, and jsp files just create HTML output so by the time a browser or Google’s spider sees the page there is no difference. If you take a look at the HTTP headers for these pages (you can do this with OptiLink) you will notice that the content type is text/html just like any "regular" HTML file. The only thing unusual is the page filename extension which bothers Google not in the least.

For really clever folks, you can’t even know what was executed on the server to produce HTML output. For example. it is trivial to configure a server to execute PHP when .html files are served — one line in an .htaccess file is all that is required, so the filename extension really and truly does not matter. The only thing that browsers and spiders can, and do, trust is the Content-type field in the HTTP header.