MDL-56110 forms: debug message for php 7.1 compat

Follow up for MDL-55123

lib/pear/HTML/QuickForm.php

@@ -544,6 +544,16 @@ function getMaxFileSize()
      */
     function &createElement($elementType)
     {
+        if (!isset($this) || !($this instanceof HTML_QuickForm)) {
+            // Several form elements in Moodle core before 3.2 were calling this method
+            // statically suppressing PHP notices. This debugging message should notify
+            // developers who copied such code and did not test their plugins on PHP 7.1.
+            // Example of fixing group form elements can be found in commit
+            // https://github.com/moodle/moodle/commit/721e2def56a48fab4f8d3ec7847af5cd03f5ec79
+            debugging('Function createElement() can not be called statically, ' .
+                    'this will no longer work in PHP 7.1',
+                    DEBUG_DEVELOPER);
+        }
         $args    =  func_get_args();
         $element = self::_loadElement('createElement', $elementType, array_slice($args, 1));
         return $element;