Add a note about PHP_FLAVOR.

Sponsored by:	Absolight

en_US.ISO8859-1/books/porters-handbook/flavors/chapter.xml

@@ -329,6 +329,14 @@ IGNORE_WITH_PHP=	72</programlisting>
 
 USES=	php:flavors</programlisting>
       </example>
+
+      <tip>
+	<para>When adding a dependency on a
+	  <application>PHP</application> flavored port, use
+	  <literal>@${PHP_FLAVOR}</literal>.
+	  <emphasis>Never</emphasis> use <varname>FLAVOR</varname>
+	  directly.</para>
+      </tip>
     </sect2>
   </sect1>