Convert numbers up to base73

base_convert_.php

<?php
/**
 * Convert a number from one base to another
 * Modelled after PHP's base_convert(), but able to handle base73
 *
 * @param string $num  The number to convert
 * @param int    $from The base from which to convert
 * @param int    $to   The base to which to convert
 *
 * @return string The converted number
 */
function base_convert_($num, $from, $to) {
	//Character set of a base-_ number
	//Characters chosen as those valid and unreserved in a URL
	//Characters higher than 62 not expected to match other base_convert implementations
	$chars = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ$-_.+!*\'(),';
	//Validate the `from' and `to' bases supplied
	if ($from < 2 || strlen($chars) < $from) {
		trigger_error("base_convert_(): Invalid `from base' ($from)", E_USER_WARNING);
		return null;
	}
	if ($to < 2 || strlen($chars) < $to) {
		trigger_error("base_convert_(): Invalid `to base' ($to)", E_USER_WARNING);
		return null;
	}

	//to mimic the functionality of PHP's base_convert, strip characts exceeding `from base'
	$num = preg_replace('/[^'.substr($chars,0,$from).']/','',strrev($num.''));

	//if `from base' is 36 or less, ignore case
	if (36 <= $from) {
		$num = strtolower($num);
	}

	//convert supplied number to an int
	$value = 0;
	for ($i = strlen($num)-1; $i >= 0; $i--) {
		$digit = strpos($chars, $num[$i]);
		if ($from <= $digit) {
			trigger_error("base_convert_(): Invalid number supplied, digit ($digit) exceeds `from base' ($from)", E_USER_WARNING);
		}
		$value += $digit * pow($from, $i);
	}

	//convert intermediate int to desired base
	$result = '';
	do {
		$result = $chars[intval($value % $to)].$result;
	} while (0 < $value = floor($value / $to));

	return $result;
}