PHP: unlink cannot delete file even the file exists and writable

I've been trying to figure out why unlink is not working. I've tried stackoverflow previous questions and answers but no luck. The exact filename that needs to be deleted is 'upload/test.png'. First I made a check if file exists.

$filename = 'upload/test.png';
if(file_exists($filename)){
// file_exists returns true
    if(is_writable($filename)){
        // is_writable also returns true
        if(unlink($filename)){
            echo 'file deleted';
        }
        else{
            echo 'cant delete file';
            print_r(error_get_last());
            // this gives me
            // unlink() function.unlink: No such file or directory
        }
    }
}

Give the full path instead, like

$filename = dirname(__FILE__) . '/upload/test.png';

Then try this,

if (is_file($filename)) {

   chmod($filename, 0777);

   if (unlink($filename)) {
      echo 'File deleted';
   } else {
      echo 'Cannot remove that file';
   }

} else {
  echo 'File does not exist';
}