MDL-18976 simplexml ext now marked as required, credit goes to Dan Poltawski

admin/environment.xml

@@ -272,6 +272,11 @@
           <ON_CHECK message="gdrecommended" />
         </FEEDBACK>
       </PHP_EXTENSION>
+      <PHP_EXTENSION name="simplexml" level="required">
+        <FEEDBACK>
+          <ON_CHECK message="simplexmlrequired" />
+        </FEEDBACK>
+      </PHP_EXTENSION>
     </PHP_EXTENSIONS>
     <PHP_SETTINGS>
       <PHP_SETTING name="memory_limit" value="40M" level="required">

lang/en_utf8/admin.php

@@ -772,6 +772,7 @@ $string['settingmemorylimit'] = 'Insufficient memory detected, please set higher
 $string['showblocksonmodpages'] = 'Show blocks on module pages';
 $string['showdetails'] = 'Show details';
 $string['simpletest'] = 'Unit tests';
+$string['simplexmlrequired'] = 'The SimpleXML PHP extension is now required by Moodle.';
 $string['dbtest'] = 'Functional DB tests';
 $string['sitelangchanged'] = 'Site language setting changed successfully';
 $string['sitemailcharset'] = 'Character set';