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).

PHP Equvilent base64_encode with javascript code

<script type="text/javascript">
function base64_encodeRsync (data) {
  var b64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
  var o1, o2, o3, h1, h2, h3, h4, bits, i = 0,
    ac = 0,
    enc = "",
    tmp_arr = [];

  if (!data) {
    return data;
  }

  do { // pack three octets into four hexets
    o1 = data.charCodeAt(i++);
    o2 = data.charCodeAt(i++);
    o3 = data.charCodeAt(i++);

    bits = o1 << 16 | o2 << 8 | o3;

    h1 = bits >> 18 & 0x3f;
    h2 = bits >> 12 & 0x3f;
    h3 = bits >> 6 & 0x3f;
    h4 = bits & 0x3f;

    // use hexets to index into b64, and append result to encoded string
    tmp_arr[ac++] = b64.charAt(h1) + b64.charAt(h2) + b64.charAt(h3) + b64.charAt(h4);
  } while (i < data.length);

  enc = tmp_arr.join('');

  var r = data.length % 3;

  return (r ? enc.slice(0, r - 3) : enc) + '==='.slice(r || 3);

}

</script>
We can decode this encrypted data in php by passing that value to  base64_decode function.