Error Saving Image in Directory - PHP/JAVASCRIPT

Question

Hello, I’m trying to finish a part of a system, where the user draws his signature and after the action, the Canvas Image is sent to the directory, but when I open the Saved image appears the following error: The image "https://br918.hostgator.com.br:2083/cpsess0222983772/view/home1%2fdsenv18%2pagina.life%2TESTE%2fpagina%2fimg/signature.png" Cannot be displayed bacause it contains error.

Could one of you help me?

PHP

$data = substr($_POST['imageData'], strpos($_POST['imageData'], ",") + 1);
$decodedData = base64_decode($data);

$fp = fopen("img/assinatura.png", 'wb');

fwrite($fp, $decodedData);
fclose($fp);

JAVASCRIPT

<script>
    function salvarImagem(a){ 

    var meucanvas = document.getElementById('newSignature');

    var arquivo = document.getElementById('arquivo');

    /*Comentário: a variavel "a" será o nome do arquivo, use aspas para chamar a função */
    arquivo.download = a;
    arquivo.href = meucanvas.toDataURL();

    function signatureSave(url, name, fn, canvas, type) {

        var data = document.getElementById("assinatura.png").toDataURL();    
        $.post("assinatura.png", {
            imageData : data
        }, function(data) {
            alert('imagem salva');
        });
    }