Uploading larger image file from javascript XMLHttpRequest blob to php (it saved wrong size)

Question

I am trying to upload larger image file 100MB to php server. It looks like a whole file uploaded (got status 200), but on server side there is wrong file size, lost few bytes, and it is not displayed as image in web browser.

How to fix?

Here is htmlp/javascript :

var file = document.getElementById("selectedfile").files[0];
var reader = new FileReader();
//reader.readAsBinaryString(file); 
reader.readAsDataURL(file);

reader.onloadend  = function(evt){

    xhr = new XMLHttpRequest();

    xhr.open("POST", "uprx.php?filename="+document.getElementById("selectedfile").files[0].name, true);

    XMLHttpRequest.prototype.mySendAsBinary = function(text){
        var data = new ArrayBuffer(text.length);
        var ui8a = new Uint8Array(data, 0);
        for (var i=0; i<text.length; i++) {
            ui8a[i] = (text.charCodeAt(i)&0xff);
        }
        if  (typeof window.Blob == "function") {
             var blob = new Blob([data]);
        }
        else {
            var bb = new (window.MozBlobBuilder || window.WebKitBlobBuilder || window.BlobBuilder)();
            bb.append(data);
            var blob = bb.getBlob();
        }

        this.send(blob);
    }

    var eventSource = xhr.upload || xhr;
    eventSource.addEventListener("progress", function(e) {

        var position = e.position || e.loaded;
        var total = e.totalSize || e.total;
        var percentage = Math.round((position/total)*100);

        
    });

    xhr.onreadystatechange = function() {
        if (xhr.readyState == 4)    {
            if (xhr.status == 200) {
                
            }
            else {
                // process error
            }
        }
    };

    xhr.mySendAsBinary(evt.target.result);
};

Here is php server :

$inputHandler = fopen("php://input", "r");

$fileHandler = fopen($_GET["filename"], "w");

while(true) {

$buffer = fgets($inputHandler, 4096);
if (strlen($buffer) == 0) {
    fclose($inputHandler);
    fclose($fileHandler);
    return true;
}

$base64Image = trim($buffer);
$base64Image = str_replace('data:image/png;base64,', '', $base64Image);
$base64Image = str_replace('data:image/jpg;base64,', '', $base64Image);
$base64Image = str_replace('data:image/jpeg;base64,', '', $base64Image);
$base64Image = str_replace('data:image/gif;base64,', '', $base64Image);
//$base64Image = str_replace(' ', '+', $base64Image);
$imageData = base64_decode($base64Image);
fwrite($fileHandler, $imageData);

}