JavaScript - Popup no funciona

<a id="md" target="popup" href="privado.php" onclick="return popitup(\''. $pluing_url.'privado.php?id_user='.$row->ID.'' .'\')" >

function popitup(url) {

  newwindow=window.open(url,'popup','height=520,width=484,top=100, left=400');

  if (window.focus) {newwindow.focus()}

  return false;

}