Task: Bitcoin_address validation
Java: import java.math.BigInteger;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.Arrays;

public class BitcoinAddressValidator {

    private static final String ALPHABET = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz";

    public static boolean validateBitcoinAddress(String addr) {
        if (addr.length() < 26 || addr.length() > 35)
            return false;
        byte[] decoded = decodeBase58To25Bytes(addr);
        if (decoded == null)
            return false;

        byte[] hash1 = sha256(Arrays.copyOfRange(decoded, 0, 21));
        byte[] hash2 = sha256(hash1);

        return Arrays.equals(Arrays.copyOfRange(hash2, 0, 4), Arrays.copyOfRange(decoded, 21, 25));
    }

    private static byte[] decodeBase58To25Bytes(String input) {
        BigInteger num = BigInteger.ZERO;
        for (char t : input.toCharArray()) {
            int p = ALPHABET.indexOf(t);
            if (p == -1)
                return null;
            num = num.multiply(BigInteger.valueOf(58)).add(BigInteger.valueOf(p));
        }

        byte[] result = new byte[25];
        byte[] numBytes = num.toByteArray();
        System.arraycopy(numBytes, 0, result, result.length - numBytes.length, numBytes.length);
        return result;
    }

    private static byte[] sha256(byte[] data) {
        try {
            MessageDigest md = MessageDigest.getInstance("SHA-256");
            md.update(data);
            return md.digest();
        } catch (NoSuchAlgorithmException e) {
            throw new IllegalStateException(e);
        }
    }

    public static void main(String[] args) {
        assertBitcoin("1AGNa15ZQXAZUgFiqJ2i7Z2DPU2J6hW62i", true);
        assertBitcoin("1AGNa15ZQXAZUgFiqJ2i7Z2DPU2J6hW62j", false);
        assertBitcoin("1Q1pE5vPGEEMqRcVRMbtBK842Y6Pzo6nK9", true);
        assertBitcoin("1AGNa15ZQXAZUgFiqJ2i7Z2DPU2J6hW62X", false);
        assertBitcoin("1ANNa15ZQXAZUgFiqJ2i7Z2DPU2J6hW62i", false);
        assertBitcoin("1A Na15ZQXAZUgFiqJ2i7Z2DPU2J6hW62i", false);
        assertBitcoin("BZbvjr", false);
        assertBitcoin("i55j", false);
        assertBitcoin("1AGNa15ZQXAZUgFiqJ2i7Z2DPU2J6hW62!", false);
        assertBitcoin("1AGNa15ZQXAZUgFiqJ2i7Z2DPU2J6hW62iz", false);
        assertBitcoin("1AGNa15ZQXAZUgFiqJ2i7Z2DPU2J6hW62izz", false);
        assertBitcoin("1Q1pE5vPGEEMqRcVRMbtBK842Y6Pzo6nJ9", false);
        assertBitcoin("1AGNa15ZQXAZUgFiqJ2i7Z2DPU2J6hW62I", false);
    }

    private static void assertBitcoin(String address, boolean expected) {
        boolean actual = validateBitcoinAddress(address);
        if (actual != expected)
            throw new AssertionError(String.format("Expected %s for %s, but got %s.", expected, address, actual));
    }
}

PHP: function validate($address){
        $decoded = decodeBase58($address);

        $d1 = hash("sha256", substr($decoded,0,21), true);
        $d2 = hash("sha256", $d1, true);

        if(substr_compare($decoded, $d2, 21, 4)){
                throw new \Exception("bad digest");
        }
        return true;
}
function decodeBase58($input) {
        $alphabet = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz";

        $out = array_fill(0, 25, 0);
        for($i=0;$i<strlen($input);$i++){
                if(($p=strpos($alphabet, $input[$i]))===false){
                        throw new \Exception("invalid character found");
                }
                $c = $p;
                for ($j = 25; $j--; ) {
                        $c += (int)(58 * $out[$j]);
                        $out[$j] = (int)($c % 256);
                        $c /= 256;
                        $c = (int)$c;
                }
                if($c != 0){
                    throw new \Exception("address too long");
                }
        }

        $result = "";
        foreach($out as $val){
                $result .= chr($val);
        }

        return $result;
}

function main () {
  $s = array(
                "1Q1pE5vPGEEMqRcVRMbtBK842Y6Pzo6nK9",
                "1AGNa15ZQXAZUgFiqJ2i7Z2DPU2J6hW62i",
                "1Q1pE5vPGEEMqRcVRMbtBK842Y6Pzo6nJ9",
                "1AGNa15ZQXAZUgFiqJ2i7Z2DPU2J6hW62I",
        );
  foreach($s as $btc){
    $message = "OK";
    try{
        validate($btc);
    }catch(\Exception $e){ $message = $e->getMessage(); }
    echo "$btc: $message\n";
  }
}

main();