document.getElementById("btnSave").addEventListener("click", event => {
    canvas.toBlob(function(blob){
        $.post('upload.php',{blob}, function(){
            alert("Foto salva com sucesso!");
        });
    });
})

$servername = "localhost";
$username = "root";
$password = "";
$database = "fotos";
// Create connection
$conn = new mysqli($servername, $username, $password, $database);
// Check connection
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}

$sql = "INSERT INTO fotos (foto) VALUES (".$_POST['imagem'].")";
$result = mysqli_query($conn, $sql) or die(mysql_error());
$row = mysqli_fetch_array($result);

mysqli_close($conn);