PHP Crop DataURL并返回DataURL

I'm trying to make a PHP script to crop a dataURL and return another dataURL, using GD Library, but I always get errors, how can I fix that ?

if(isset($_GET['render'])) {
    if((isset($_GET['render_x'])) && (isset($_GET['render_y']))) {
        if(isset($_GET['dataURL'])) {
            $image = $_GET['dataURL']; // the image to crop
            $image = substr($image,22);
            $img = imagecreatetruecolor('200','150');
            $org_img = imagecreatefromstring(base64_decode($image));
            imagecopy($img,$org_img, 0, 0, 20, 20, 200, 150);
            ob_start(); 
            imagepng($img);
            $image_data = ob_get_contents(); 
            ob_end_clean (); 
            $image_data_base64 = base64_encode($image_data);
            imagedestroy($img);
            echo '<img src="data:image/png;base64,'.$image_data_base64.'" ><p>';
        }
    }
}

How can I fix that ? I got these errors :

Warning: imagecreatefromstring(): gd warning: one parameter to a memory allocation multiplication is negative or zero, failing operation gracefully in xxx on line 29

Warning: imagecreatefromstring(): Passed data is not in 'WBMP' format in xxx on line 29

Warning: imagecreatefromstring(): Couldn't create GD Image Stream out of Data in xxx on line 29

Warning: imagecopy() expects parameter 2 to be resource, boolean given in

and a black picture
EDIT : $_GET['dataURL'] value: data:image/png;base64,AAAFBfj42Pj4
How can I fix that ? Thanks

I think you should leave out the substr part in your code. Because the image data (type of image) is important too.

Also is the length of your url limited to an certain amount of characters per browser. So to be save keep it under 2000 characters. If that's not possible. Get the content of the image by post as @tacone suggested in his comment.

if(isset($_GET['render'])) {
    if((isset($_GET['render_x'])) && (isset($_GET['render_y']))) {
        if(isset($_GET['dataURL'])) {
            $image = $_GET['dataURL']; // the image to crop
            //$image = substr($image,22);
            $img = imagecreatetruecolor('200','150');
            $org_img = imagecreatefromstring(base64_decode($image));
            imagecopy($img,$org_img, 0, 0, 20, 20, 200, 150);
            ob_start(); 
            imagepng($img);
            $image_data = ob_get_contents(); 
            ob_end_clean (); 
            $image_data_base64 = base64_encode($image_data);
            imagedestroy($img);
            echo '<img src="data:image/png;base64,'.$image_data_base64.'" ><p>';
        }
    }
}