php Fatal error: Call to undefined method TCPDF::addTTFfont() in

In TCPDF (Version 6.2.6) working with:

require_once('/your_path_to/tcpdf.php');
$pdf = new TCPDF(PDF_PAGE_ORIENTATION, PDF_UNIT, PDF_PAGE_FORMAT, true, 'UTF-8', false);
$fontname = TCPDF_FONTS::addTTFfont('../common/pdf/fonts/persianFont/HiwebNazanin.ttf', 'TrueTypeUnicode', '', 32);