How can you encrypt a url in php

 <? While($rs = mysql_fetch_assoc( $r ) ){ ?>
<a href="index2_content.php?id=<?=$rs[‘id’]?>"><?=$rs['aux']?></a>
<? } ?>
<? While($rs = mysql_fetch_assoc( $r ) ){ ?>
<a href="noticias_detalle.php?id=<?=md5($key.$rs['id'])?>"><?=$rs['aux']?></a>
<?} ?>
                <? echo '<a class="navbar-brand" href="index2_content.php?id='.$aux.'">'?>
                    <img src="logo_movistar.png"  onclick="waitingDialog.show();setTimeout(function () {waitingDialog.hide();}, 3000);"  alt="">


                </a>
            </div>