Convert HEXA to decimal

hex2dec.php

<?php
function hexTDec($hex)
{
    $decimal = 0;
    $exp = 0;
    $len = strlen($hex) - 1;
    for ($i = $len; $i >= 0 ; $i--) { 
        if ($hex[$i] <= 9 && $hex[$i] >= 0){
            $decimal += $hex[$i] * ( 16 ** $exp);
        } 
        if ($hex[$i] <= "F" && $hex[$i] >= "A") {
            $decimal += (ord($hex[$i]) - 55) * (16 ** $exp);
        }
        $exp++;
    }
    return $decimal;
}
//Usage
echo hexTDec("123");