Save signed image inside the server - PHP

Question

I'm using this code to draw on the screen https://github.com/szimek/signature_pad

How to save within the folder of the server the image created from the markings along with the background image.

Sample directory: /home4/host/public_html/testes/

var signaturePad = new SignaturePad(document.getElementById('signature-pad'), {
  backgroundColor: 'rgba(255, 255, 255, 0)',
  penColor: 'rgb(0, 0, 0)'
});
var saveButton = document.getElementById('save');
var cancelButton = document.getElementById('clear');

saveButton.addEventListener('click', function(event) {
  var data = signaturePad.toDataURL('image/png');

  // Send data to server instead...
  window.open(data);
});

cancelButton.addEventListener('click', function(event) {
  signaturePad.clear();
});

.wrapper {
  position: relative;
  width: 250px;
  height: 367px;
  -moz-user-select: none;
  -webkit-user-select: none;
  -ms-user-select: none;
  user-select: none;
}

img {
  position: absolute;
  left: 0;
  top: 0;
}

.signature-pad {
  position: absolute;
  left: 0;
  top: 0;
  width: 250px;
  height: 367px;
}

<script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/signature_pad.umd.min.js"></script>


<div class="wrapper">
  <img src="http://www.hostcia.net/testes/html5Assinatura/Hatch.jpg" width=250 height=367 />
  <canvas id="signature-pad" class="signature-pad" width=250 height=367></canvas>
</div>
<div>
  <button id="save">Save</button>
  <button id="clear">Clear</button>
</div>

Thank you.