takes a screenshot from some html element with html2canvas.js and save it to server with PHP.

html2canvas.js

html2canvas(document.getElementById("an_element_id"), {
    onrendered: function(canvas) {
        var img = canvas.toDataURL("image/png");
        $.post("http://blah.com", {image_data: img}, function (data, textStatus, jqXHR) {
            console.log(arguments);
        });
    }
});

save_canvas_image.php

<?php

$imageData = $_POST["image_data"];

if (empty($imageData)) {
    header($_SERVER["SERVER_PROTOCOL"] . " 500 Internal Server Error", true, 500); exit();
}

$imageBase64 = str_replace("data:image/png;base64,", "", $imageData);
$imageBin = base64_decode($imageBase64);
file_put_contents(dirname(__FILE__) . "/anImage.png", $imageBin, LOCK_EX);