The XML and XSLT functions in PHP handle entities in the following manner: <ol><li>DOM document loads the actual XSL as an XML document first. As this point, the DOM object can substituteEntities declared in the doctype for you.</li><li>After that, the XsltProcessor will not substitute entities without a DTD handy.</li><li>When the XsltProcessor encounters an import or include function, it will load the additional xsl stylesheet, but if it encounters an entity that has not been declared via a DTD, it will throw a fatal error.</li></ol>

Therefore, don’t put entities in your included stylesheets. There may be a way that I am unaware of, and I’m pretty much posting this as a reminder to myself.

Related Info:

PHP.net comments 54777 (XML Catalogs for speedy DTD Resolution), 57274 (Local storage of XML entity catalogs), and Whump.com XML Catalogs are very important - great post - references not only Marc Liyanage, but also Christian Stocker (even though Marc’s name is spelled wrong!).