gistfile1.php

<?php

$GLOBALS['TL_DCA']['tl_content']['fields']['my_headline']['save_callback'][] = array('Hooks', 'saveMyHeadline');

class Hooks
{
    public function saveMyHeadline($value, $dc)
    {
        $headline = array('value' => $value, 'unit' => 'h2');
        
        $database = \Database::getInstance();
        $database
            ->prepare('UPDATE tl_content SET headline=? WHERE id=?')
            ->execute($headline, $dc->id);
        
        return $value;
    }
}