foreach($aTag as $sKey => $sValue)
{
	$fTotal += $sValue*(1/pow(22, $sKey+1));
}
$tag = $fTotal;

Here's a function that works with negative powers:

<?php
function newpow($base, $power)
{
if ($power < 0) {
$npower = $power - $power - $power;
return 1 / pow($base, $npower);
}
else
{
return pow($base, $power);
}
}
?>

Here's a pow() function that allows negative bases :

<?php
function npow($base, $exp)
{
    $result = pow(abs($base), $exp);
    if ($exp % 2 !== 0) {
        $result = - ($result);
    } 
    return $result;
} 
?>