如何使用php和ajax将Canvas元素保存到png文件？

I want to save a html canvas element as an image using php and jquery ajax. Here is my code for ajax.

   var  front_image=canvas.toDataURL("image/png");
//front image is a base_64 string
                 $.ajax({
                  url:base_url+'tabs/profile/save_front_image',
                  type:'POST',
                  data:'front_image='+front_image,
                  success:function(response){


                  }
              });

I m just doing echo in php echo $_POST['front_image'] so request and response are same.

When i use this code before ajax it loads image to new tab of browser

 var w = window.open('about:blank', 'image from canvas');
   w.document.write("<img src='" + frame_image + "' alt='from canvas'/>");

but when i put the same code on ajax response as bellow it doesn't work. Only a blank tab opens in browser. So i m not being able to save image as file.

var w = window.open('about:blank', 'image from canvas');
 w.document.write("<img src='" + response + "' alt='from canvas'/>");

I compared string length of frame_image and response also. They are same. I m not sure why image is not loading in response. Please suggest me the answer thanks.

// soon you can use front_image=canvas.toBlob("image/png")

// construct a blob
function b64toBlob(b64Data, contentType, sliceSize) {
    contentType = contentType || '';
    sliceSize = sliceSize || 512;

    var byteCharacters = atob(b64Data);
    var byteArrays = [];

    for (var offset = 0; offset < byteCharacters.length; offset += sliceSize) {
        var slice = byteCharacters.slice(offset, offset + sliceSize);

        var byteNumbers = new Array(slice.length);
        for (var i = 0; i < slice.length; i++) {
            byteNumbers[i] = slice.charCodeAt(i);
        }

        var byteArray = new Uint8Array(byteNumbers);

        byteArrays.push(byteArray);
    }

    var blob = new Blob(byteArrays, {type: contentType});
    return blob;
}

// make an actually file from the base64 so we can send binary data
var blob = b64toBlob(front_image.split(",")[1], "image/png")
var fd = new FormData();

fd.append("file", blob, "filename.png");

$.ajax({
    url: 'http://example.com/script.php',
    data: fd,
    processData: false,
    contentType: false,
    type: 'POST',
    success: function(data){
        alert(data);
    }
});

// The saving

<?php
$uploads_dir = '/uploads';
foreach ($_FILES["file"]["error"] as $key => $error) {
    if ($error == UPLOAD_ERR_OK) {
        $tmp_name = $_FILES["file"]["tmp_name"][$key];
        $name = $_FILES["file"]["name"][$key];
        move_uploaded_file($tmp_name, "$uploads_dir/$name");
    }
}
?>