How to put HTML data into header of tcpdf

As @vinzcoco says, you must extend TCPDF to achieve what you want. Here is a simple improvement that I think it could be useful for you:

class MyTCPDF extends TCPDF {

    var $htmlHeader;

    public function setHtmlHeader($htmlHeader) {
        $this->htmlHeader = $htmlHeader;
    }

    public function Header() {
        $this->writeHTMLCell(
            $w = 0, $h = 0, $x = '', $y = '',
            $this->htmlHeader, $border = 0, $ln = 1, $fill = 0,
            $reseth = true, $align = 'top', $autopadding = true);
    }

}

Now, once you've got your MyTCPDF object available, you just need to do this to set the HTML header content:

$mytcpdfObject->setHtmlHeader('<table>...</table>');

and the HTML content won't be hardcoded into the Header() method (more flexible for you).