[laimas]

<a href="https://wa.me/<?=$whatsapp?>" class="whatsapp" title="Whatsapp"><?=str_replace(['+','-','(',')'], '', $whatsapp)?></a>

или

<a href="https://wa.me/<?=$whatsapp?>" class="whatsapp" title="Whatsapp"><?=preg_replace('/\D/', '', $whatsapp)?></a>