Как использоваться CURLOPT_PASSWDFUNCTION?

function get_passwd($ch, $string, $length){
    return 'password';
}
curl_setopt($ch, CURLOPT_PASSWDFUNCTION, 'get_passwd');

PHP Notice:  Use of undefined constant CURLOPT_PASSWDFUNCTION - assumed 'CURLOPT_PASSWDFUNCTION' in

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