How to save captured photo in php variable?

I get the picture through javascript, however I can not save it in a php variable

<?php
  if (!isset($_SESSION['reglogin'])){
    header("location: index.php");
  }else{
    if(isset($_POST['canvas'])){
      $_SESSION['mfoto'] = $_POST['canvas'];
      print_r($_SESSION['mfoto']);
    }
?>
  <input/ type="hidden" name="acao" id="acao" value="cad_selfie">
    <div id="site_contents">
      <div class="login-logo">
        <img src='images/logotipo.jpg' width="100%" alt='GSR Money' title='' border='' onClick='' />
        <img src='images/linhaCadastro_3.jpg' width="100%" alt='GSR Money' title='' border='' onClick='' />
      </div>

      <div id="loginpanelwrap_selfie">

        <div class="loginheader">
          <div>
            <div>
              <table>
                <tr align="left">
                  <td style="color: red">*Tire uma foto com o documento aberto na m&atilde;o</td>
                </tr>
              </table>

              <video id="video" width="600" height="440" autoplay></video>
            </div>
            <div>
              <button id="snap">Tirar Foto</button>
            </div>

            <div>
                <canvas id="canvas" width="600" height="440"></canvas>
            </div>

            <table>
              <tr>
                <td>&nbsp;</td>
              </tr>
            </table>
            <table>
              <tr>
                <td width="400px">&nbsp;</td>
                <td>
                  <input type="button" onclick="funcaoContinuar();" class="form_submit_continuar_azul" name="btncontinuar" id="btncontinuar" value="Continuar" />
                </td>
              </tr>
            </table>
          </div>
          <div id="tenta" class="clear"></div>
          <?php
                if(isset($_SESSION['imgfoto'])){
                    print_r($_SESSION['imgfoto']);
                }
                ?>
        </div>
      </div>
      <script>
        window.addEventListener("DOMContentLoaded", function() {
          var canvas = document.getElementById("canvas"),
            context = canvas.getContext("2d"),
            video = document.getElementById("video"),
            videoObj = {
              "video": true
            },
            errBack = function(error) {
              console.log("Video capture error: ", error.code);
            };
          if (navigator.getUserMedia) {
            navigator.getUserMedia(videoObj, function(stream) {
              video.src = stream;
              video.play();
            }, errBack);
          } else if (navigator.webkitGetUserMedia) {
            navigator.webkitGetUserMedia(videoObj, function(stream) {
              video.src = window.webkitURL.createObjectURL(stream);
              video.play();
            }, errBack);
          } else if (navigator.mozGetUserMedia) {
            navigator.mozGetUserMedia(videoObj, function(stream) {
              video.src = window.URL.createObjectURL(stream);
              video.play();
            }, errBack);
          }
        }, false);
        document.getElementById("snap").addEventListener("click", function() {
          canvas.getContext("2d").drawImage(video, 0, 0, 600, 440);
          //alert(canvas.toDataURL());

        });

        function funcaoContinuar() {
          document.frm_selfie.submit()
        }
      </script>
      <form name="frm_selfie" id="frm_selfie" method="post">
        <input/ type="hidden" name="acao" id="acao" value="cad_rg">
          <!-- EU PRECISO SALVAR A IMAGEM DENTRO DESSE FORM PORQUE NA PRÓXIMA PÁGINA EU QUERO MOSTRAR A IMAGEM E NISSO O CLIENTE
            FINALIZARÁ O CADASTRO E VOU SALVAR A IMAGEM NO BANCO MYSQL
             -->
      </form>
      <?php
    }
    ?>