如何获取data_uri并将其放入变量中

Question

I have this javascript that takes a snapshot and echo it the image, the image is base64 so I need to change it into a proper file but I don't know how here's my javascript

<script src="webcam.js"></script> <!--source code script from github for webcam config-->

    <div id="my_camera" style="width:320px; height:240px;"></div>
    <div id="my_result"></div>

    <script type=text/javascript>
        Webcam.attach( '#my_camera' );

        function take_snapshot() {
            Webcam.snap( function(data_uri) {

                document.getElementById('my_result').innerHTML = '<img src="'+data_uri+'"/>';

            } );
        }
    </script>

this is how to decode base on the PHP documentation

  <?php
$str = 'VGhpcyBpcyBhbiBlbmNvZGVkIHN0cmluZw==';
echo base64_decode($str);
?>

but I don't know the process on how to declare the data_uri from my javascript and put it into a $str or a variable and change it into a file without actually saving it and put it into this function

<?php
if ($_SERVER['REQUEST_METHOD'] != "POST"){
    header("location:take-attendance.php");
    die();
}

require "vendor/autoload.php";

$qrcode = new Zxing\QrReader($_FILES['qrimage']['tmp_name']);
$text = $qrcode->text();

var_dump($text);

?>

I want to get a snapshot of a QR Code then decode that snapshot, thanks in advance