<?php
function FormataCEP($cep) {
    $cep = ereg_replace("[^0-9]", "", $cep);
    if (strlen($cep) == 8) return $cep[0] . $cep[1] . "." . $cep[2] . $cep[3] . $cep[4] . '-'. $cep[5] . $cep[6] . $cep[7]; 
    else return false;
}

echo FormataCEP('8153aaaa0a510');
?>