php random file name issue

I'm uploading the canvas tables to the specified folder for my project. the files are saved in the folder I specified with a random file name. I want to make the names of these files a fixed name.

<script>
    function uploadEx(data, isim) {
        id = data;
        var canvas = document.getElementById('canvas'+id);
        var dataURL = canvas.toDataURL('image/png');
        document.getElementById('hidden_data').value = dataURL;
        var fd = new FormData(document.forms["form1"]);

        var xhr = new XMLHttpRequest();
        xhr.open('POST', 'upload_data.php', true);

        xhr.upload.onprogress = function(e) {
            if (e.lengthComputable) {
                var percentComplete = (e.loaded / e.total) * 100;
                console.log(percentComplete + '% uploaded');
               //alert('Succesfully uploaded');
            }
        };

        xhr.onload = function() {

        };
        xhr.send(fd);
    };
</script>

upload_data.php

$upload_dir = "upload/";
$img = $_POST['hidden_data'];
$img = str_replace('data:image/png;base64,', '', $img);
$img = str_replace(' ', '+', $img);
$data = base64_decode($img);
$file = $upload_dir . mktime() . ".png";
$success = file_put_contents($file, $data);
print $success ? $file : 'Unable to save the file.';