Please tell me why this form of redirect does not work?
if ($mail->Send()) {
return '<span class="sppb-text-success">'. JText::_('COM_SPPAGEBUILDER_ADDON_AJAX_CONTACT_SUCCESS') .'</span>';
header('Refresh: 3; URL=index.html');
} else {
return '<span class="sppb-text-danger">'. JText::_('COM_SPPAGEBUILDER_ADDON_AJAX_CONTACT_FAILED') .'</span>';
}