<input type="submit"  name="submit_s"  style=" background-image:URL(img/search.gif); background-repeat:no-repeat;" >

<form action="pr_search.php"  method="post">
      <input name="search" type="text" class="search_input" value=" поиск..." size="40%" maxlength="50" />
  	  <input type="image" src="img/search.gif" name="submit_s" value="OK" >
        <a href="#" class="search_pk" >Как искать?</a>
        </form>

<?php
$db = mysql_connect ("localhost","root");
mysql_select_db("abc",$db);
mysql_query("SET CHARACTER SET cp1251");
mysql_query("SET NAMES cp1251");


if (isset($_POST['sub_search'])) {$sub_search = $_POST['sub_search'];}
if (isset($_POST['search'])) {$search = $_POST['search'];}

if(isset($sub_search)) 
{
if (empty($search) or strlen($search) < 3) 
{exit("<p>Поисковой запрос не введен , либо он менее 4 символов.</p>");}
$search = trim($search);
$search = htmlspecialchars($search);


}
else 
{
exit("<p>Вы обратились к файлу без необходимых параметров</p>");
}

  ?>

 <input type="image" src="img/search.gif" name="sub_search" value="OK" >