Here's what finally worked for me. You'll have to convert the code to suit your own needs, but this will do it.
$fname = filter_input(INPUT_POST, "name");
$img = filter_input(INPUT_POST, "image");
$img = str_replace('data:image/png;base64,', '', $img);
$img = str_replace(' ', '+', $img);
$img = base64_decode($img);
file_put_contents($fname, $img);
print "Image has been saved!";