Save canvas with an image as PNG works only in Firefox - php

Question

I have an canvas on my site where the user can draw and can also add an image. By clicking a Save button I want the canvas to be saved as a PNG image.
Everything works fine in Firefox, but in Chrome for example when I add an image in the canvas and then try to save it, the image on the canvas has not been saved.
My code in javascript for adding an image is:
var imageObj = new Image();
imageObj.onload = function(){
x.drawImage(this, destX, destY, destWidth, destHeight);
};
imageObj.src = 'myImage.png';
Then for saving the canvas:
var drawingString = myDrawing.toDataURL("image/png");
var postData = "canvasData="+drawingString;
var ajax = new XMLHttpRequest();
ajax.open("POST",'save_image.php',true);
<!-- set the mime type so the image goes through as base64 -->
ajax.setRequestHeader('Content-Type', 'canvas/upload');
ajax.onreadystatechange=function() {
<!-- once the image data has been sent -->
if (ajax.readyState == 4){
alert("Saved");
}
}
ajax.send(postData);
and in PHP:
if (isset($GLOBALS["HTTP_RAW_POST_DATA"]))
{
// Get the data like you would with traditional post
$rawImage=$GLOBALS['HTTP_RAW_POST_DATA'];
// Remove the headers
$removeHeaders=substr($rawImage, strpos($rawImage, ",")+1);
// decode it from base 64 and into image data only
$decode=base64_decode($removeHeaders);
// save to your server
$imageFile = 'myImage.png';
$fopen = fopen($imageFile, 'w' );
fwrite( $fopen, $decode);
fclose( $fopen );
}
The problem is that the background canvas it is saved in both cases. Only the image on top of the background is note saved in Chrome, Safari or IE. Does anyone know why this code is behaved differently in Firefox and in other browsers?