I found these 2 scripts online:
Javascript:
<html>
<head>
<script type="text/javascript" src="http://dl.dropboxusercontent.com/u/28441300/BigInt.js"></script>
<script type="text/javascript" src="http://dl.dropboxusercontent.com/u/28441300/sha256.js"></script>
</head>
<body>
<div id="text">
</div>
<script type="text/javascript">
var address = "1Eym7pyJcaambv8FG4ZoU8A4xsiL9us2zz";
if (check(address)) {
document.getElementById('text').innerHTML += "valid";
} else {
document.getElementById('text').innerHTML += "invalid";
}
function check(address) {
var decoded = base58_decode(address);
if (decoded.length != 25) return false;
var cksum = decoded.substr(decoded.length - 4);
var rest = decoded.substr(0, decoded.length - 4);
var good_cksum = hex2a(sha256_digest(hex2a(sha256_digest(rest)))).substr(0, 4);
if (cksum != good_cksum) return false;
return true;
}
function base58_decode(string) {
var table = '123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz';
var table_rev = new Array();
var i;
for (i = 0; i < 58; i++) {
table_rev[table[i]] = int2bigInt(i, 8, 0);
}
var l = string.length;
var long_value = int2bigInt(0, 1, 0);
var num_58 = int2bigInt(58, 8, 0);
var c;
for(i = 0; i < l; i++) {
c = string[l - i - 1];
long_value = add(long_value, mult(table_rev[c], pow(num_58, i)));
}
var hex = bigInt2str(long_value, 16);
var str = hex2a(hex);
var nPad;
for (nPad = 0; string[nPad] == table[0]; nPad++);
var output = str;
if (nPad > 0) output = repeat("\0", nPad) + str;
return output;
}
function hex2a(hex) {
var str = '';
for (var i = 0; i < hex.length; i += 2)
str += String.fromCharCode(parseInt(hex.substr(i, 2), 16));
return str;
}
function a2hex(str) {
var aHex = "0123456789abcdef";
var l = str.length;
var nBuf;
var strBuf;
var strOut = "";
for (var i = 0; i < l; i++) {
nBuf = str.charCodeAt(i);
strBuf = aHex[Math.floor(nBuf/16)];
strBuf += aHex[nBuf % 16];
strOut += strBuf;
}
return strOut;
}
function pow(big, exp) {
if (exp == 0) return int2bigInt(1, 1, 0);
var i;
var newbig = big;
for (i = 1; i < exp; i++) {
newbig = mult(newbig, big);
}
return newbig;
}
function repeat(s, n){
var a = [];
while(a.length < n){
a.push(s);
}
return a.join('');
}
</script>
</body>
</html>
PHP:
<?php
function checkAddress($address)
{
$origbase58 = $address;
$dec = "0";
for ($i = 0; $i < strlen($address); $i++)
{
$dec = bcadd(bcmul($dec,"58",0),strpos("123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz",substr($address,$i,1)),0);
}
$address = "";
while (bccomp($dec,0) == 1)
{
$dv = bcdiv($dec,"16",0);
$rem = (integer)bcmod($dec,"16");
$dec = $dv;
$address = $address.substr("0123456789ABCDEF",$rem,1);
}
$address = strrev($address);
for ($i = 0; $i < strlen($origbase58) && substr($origbase58,$i,1) == "1"; $i++)
{
$address = "00".$address;
}
if (strlen($address)%2 != 0)
{
$address = "0".$address;
}
if (strlen($address) != 50)
{
return false;
}
if (hexdec(substr($address,0,2)) > 0)
{
return false;
}
return substr(strtoupper(hash("sha256",hash("sha256",pack("H*",substr($address,0,strlen($address)-8)),true))),0,8) == substr($address,strlen($address)-8);
}
?>
Source: http://www.php.net/manual/en/ref.bc.php
http://jsfiddle.net/timrpeterson/XsCQq/2/