Decompress bitcoin public keys (PHP)

gistfile1.txt

// Decompressing a compressed bitcoin public key.
// Will require a big number library, with functions for modular exponent, square root (mod prime), and modulus.
// Requires gmp, and mathyass danters ecclib

public static function decompress_public_key($key)
{
  $y_byte = substr($key, 0, 2);
  $x_coordinate = substr($key, 2);

  // Convert x hexadecimal to a decimal
  $x = gmp_strval(gmp_init($x_coordinate, 16),10);
  $curve = \SECcurve::curve_secp256k1();
  $generator = \SECcurve::generator_secp256k1();

  try
  {
    // x^3 % p
    $x3 = \NumberTheory::modular_exp( $x, 3, $curve->getPrime() );

    // y^2 = x^3 + b
    $y2 = gmp_add(
      $x3,
      $curve->getB()
    );

    // Solve for y:
    // y = (x^3 + b)^(1/2) % p
    $y0 = \NumberTheory::square_root_mod_prime(
      gmp_strval($y2, 10),
      $curve->getPrime()
    );

    // Could be another y. Prefix specifies if value is even or odd.
    // y' = p - y
    $y1 = gmp_strval(gmp_sub($curve->getPrime(), $y0), 10);

    // if prefix is 02: 
    //    if y%2 == 0: 
    //       real_y = y
    //    else 
    //       real_y = y'
    // if prefix is 03: 
    //    if y%2 !== 0:
    //       real_y = y
    //    else
    //       real_y = y'
    $y_coordinate = ($y_byte == '02')
      ? ((\gmp_Utils::gmp_mod2(gmp_init($y0, 10), 2) == '0') ? $y0 : $y1)
      : ((\gmp_Utils::gmp_mod2(gmp_init($y0, 10), 2) !== '0') ? $y0 : $y1);


    // Convert the y coordinate to hex, and pad it to 64 characters.
    $y_coordinate = str_pad(gmp_strval($y_coordinate, 16),64,'0',STR_PAD_LEFT);

    // Instantiate a point based on curve, x coordinate, y coordinate, and order of the curve.. To test that the decompressed point is valid.
    $point = new \Point($curve, gmp_strval(gmp_init($x_coordinate, 16),10), gmp_strval(gmp_init($y_coordinate, 16),10), $generator->getOrder());
  }
  catch (\Exception $e)
  {
    return FALSE;
  }

  return array(
    'x' => $x_coordinate,
    'y' => $y_coordinate,
    'point' => $point,
    'public_key' => '04'.$x_coordinate.$y_coordinate);
}