<form action="" method="post">
      <input type="text" maxlength="15" size="15" name="dana" />
      <input type="submit" value="OK"/>
     </form>
    
    
    <?php
    function sprawdzenie($dana)
     {
      if (eregi('[a-zA-Z0-1]+$', $dana))
      {
      return true;
      }
      else
      {
      return false;
      }
     }
    
      if (sprawdzenie($_POST['dana']))
       {
       echo 'ok';
       $foo = $_POST['dana']; //a po tem do bazy albo co dusza zapragnie :)
       }
       else
       {
       echo 'zle';
       }
    
    ?>