generating Unique random key in php

function get_cripta()
{
$year = date('Y');
$month = date('m');
$day = date('d');
$hour = date('H');
$min = date('i');
$sec = date('s');
$coef = rand(1,99999);
$quo1 = ($year*$month*$day);
$quo2 = ($hour+$min+$sec);
$result = ($quo1*$quo2)*$coef;
$final = md5($result);
return $final;
}