static string calculaMD5(string cadena)
        {
            MD5CryptoServiceProvider md5 = new MD5CryptoServiceProvider();
            byte[] bytValue = md5.ComputeHash(Encoding.Default.GetBytes(cadena));
            StringBuilder sBuilder = new StringBuilder();
            for (int i = 0; i < bytValue.Length; i++)
            {
                sBuilder.Append(bytValue[i].ToString("x2"));
            }
            return sBuilder.ToString().ToUpper();

        }

<?php
  $cadena=...
  $md5= md5($cadena,true);
  echo  bin2hex( $md5 );

?>