Write and read data to php with AddBinaryData

I am trying to save and load byte[] data to php.

My functions in Unity are;

 private function UploadData(data: byte[]) {
 
     var url: String = "mywebsite.com/upload.php";
     var form: WWWForm = new WWWForm();
     form.AddBinaryData("data", data);
     var www: WWW = new WWW(url, form);
     yield www;
     if (www.error != null) {
         print(www.error);
     }
     else {
         print("Finished Uploading");
     }
     www.Dispose();
     
 }


 private function DownloadData(data: byte[]) {
     
     var url: String = "mywebsite.com/download.php";
     var form: WWWForm = new WWWForm();
     form.AddField("FileName", "test.xyx");
     var www: WWW = new WWW(url, form);
     yield www;
     if (www.error != null) {
         print(www.error);
     }
     else {
         print("Got object");
         var loadBytes: byte[] = www.bytes;
     }
     www.Dispose();
     
 }

upload.php is;

 <?php
     $mydata = ($_FILES["data"]);
     $fileName = "test.xyx";
      
     $fh = fopen($fileName , 'wb');
     fwrite($fh, $mydata);
     fclose($fh);
 ?>

download.php is;

 <?php
 
     $FileName = $_POST['FileName'];
 
     $handle = fopen($FileName , "rb");
     $data = fread($handle, filesize($FileName ));
     fclose($handle);
 
     echo $data;
 ?>

The binary I get back does not match what I send. I am very new to php so any help would be great