Problem with form builder

*I recive this message to test form builder.. Fatal error*: Call to undefined function: is_a() in */usr/share/php/DB/DataObject/FormBuilder.php* on line *289 --->php file <?php include_once("DB/DataObject.php"); $config = parse_ini_file('../DataObjects/imobiliaria.ini',TRUE); foreach($config as $class=>$values) {

       $options = &PEAR::getStaticProperty($class,'options');
               $options = $values;
               }

include_once("../class/Clientes.php"); include_once("DB/DataObject/FormBuilder.php"); $do =& new Cliente(); // Insert "$do->get($some_id);" here to edit an // existing object instead of making a new one $fg =& DB_DataObject_FormBuilder::create($do); $form =& $fg->getForm(); if ($form->validate()) { $form->process(array(&$fg,'processForm'), false); $form->freeze(); } $form->display(); ?> --->end file Error is here on formbuider ---> function useForm(&$form, $append=false) {

       if (is_a($form, 'html_quickform') && !is_object($this->_form)) {
           $this->_form =& $form;
           $this->_appendForm = $append;
           return true;
       }
       return false;

} --->> I use : root@kurumin:/usr/share/php/DB/DataObject# php -vresion PHP 4.3.4 (cli) (built: Mar 27 2004 08:04:22) Copyright (c) 1997-2003 The PHP Group Zend Engine v1.3.0, Copyright (c) 1998-2003 Zend Technologies What's happen ? *