PHP Warning: Use of undefined constant

Сервер постоянно логирует вот такую ошибку:

PHP Warning: PHP Warning: Use of undefined constant cz2_customer_form_callback - assumed 'cz2_customer_form_callback' (this will throw an Error in a future version of PHP) in /home/jadech/www/jade.ch/wp-content/plugins/clubzone/plugin.php on line 33

function cz2_customer($text) {
return preg_replace_callback("|\[czCustomer(.*)\]|", cz2_customer_form_callback, $text); }
function cz2_customer_form_callback($options) {
$config = array();

if (count($options) == 2) {
    $options = explode(" ", $options[1]);
    for ($i = 0; $i < count($options); $i++) {
        if (empty($options[$i])) continue;
        list($k, $v) = explode("=", trim($options[$i]));
        $config[$k] = $v;
    }
}

echo cz2_customer_form(array("form") + $config);

}