apache2.xml Add notes about FallbackResource

install/unix/apache2.xml

@@ -265,6 +265,34 @@ LoadModule php7_module modules/libphp7.so
     </programlisting>
    </informalexample>
 
+   <para>
+    To allow use of a PHP file as the default handler if no other handler is found,
+    for example when using a routing engine, the <literal>FallbackResource</literal> directive may be used.
+    This is available in Apache 2.4.4 and later.
+   </para>
+
+   <para>
+    Because the <literal>SetHandler</literal> directive is applied whether the file exists or not, and the
+    <literal>FallbackResource</literal> is only applied if a handler has not already been set,
+    you may need to use the <literal>FilesMatch</literal> directive to ensure that the handler is only
+    applied if the file exists.
+    This allows the <literal>FallbackResource</literal> to handle paths which end in <literal>.php</literal>
+    but do not exist, which may be useful for error handling and routing.
+   </para>
+
+   <informalexample>
+    <programlisting role="apache-conf">
+<![CDATA[
+<FilesMatch "\.php$">
+    <If "-f %{REQUEST_FILENAME}">
+        SetHandler application/x-httpd-php
+    </If>
+</FilesMatch>
+FallbackResource /index.php
+]]>
+    </programlisting>
+   </informalexample>
+
    <para>
     <literal>mod_rewrite</literal> may be used to allow any arbitrary <literal>.php</literal> file to be displayed
     as syntax-highlighted source code, without having to rename or copy it