PHP 函数 is_subclass_of()

<?php
   // define a class
   class wid_fact {
      var $oink = 'moo';
   }

   // define a child class
   class wid_fact_child extends wid_fact {
      var $oink = 'oink';
   }

   // create a new object
   $WF = new wid_fact();
   $WFC = new wid_fact_child();

   if (is_subclass_of($WFC, 'wid_fact')) {
      echo "yes, \$WFC is a subclass of wid_fact \n";
   }else {
      echo "no, \$WFC is not a subclass of wid_fact \n";
   }

   if (is_subclass_of($WF, 'wid_fact')) {
      echo "yes, \$WF is a subclass of wid_fact \n";
   }else {
      echo "no, \$WF is not a subclass of wid_fact \n";
   }
?>