How to vertically center text with mpdf

Howto center text vertically and horizontally on a single page in mpdf

(I know the answer, stackoverflow just makes me write a longer question to be allowed to post it with my answer here ...)

Solution

While mpdf supports some css, a couple of things that would allow you to center via CSS are not working:

display: flex is not working
<table> with height: 100% is not working
<div style="position: absolute; top: 50%; left: 50%;"></div> is not working

But, I did find one trick: $mpdf->hPt and $mpdf->wPt return the pages dimensions in points.
Which means you can use a table cell with height: {$mpdf->hPt}pt; vertical-align: middle; text-align: center;:

$mpdf = new \Mpdf\Mpdf($options);

$h = $mpdf->hPt;
$w = $mpdf->wPt;

$html = <<<HTML
<html>
<body style="margin: 0; padding: 0;">
<table style="width: {$w}pt; margin: 0; padding: 0;" cellpadding="0" cellspacing="0">
  <tr>
    <td style="height: {$h}pt; text-align: center; vertical-align: middle; padding: 0px 5px; margin: 0;">
      Hello World
    </td>
  </tr>
</table>
</body>
</html>
HTML;

$mpdf->WriteHTML($html);

Obviously, using the dpi to calculate that value yourself would be an option, but using mpdfs calculation ensures you have the same rounding and handling of edge cases.