Plus med php/mysql

PHP kode 

 <?php
 if (is_numeric($_POST['plus']) && !empty($_POST['plus'])) {
     $conn = new PDO('mysql:host=localhost;dbname=*testdb;charset=utf8', '*username', '*password');
     $stmt = $conn->query('SELECT * FROM *table'); 
     while($row = $stmt->fetch()) {
         $plus = $row['*field1'] + $_POST['plus'];
         $stmt2 = $conn->prepare("UPDATE *table SET *field1=:plus WHERE *field2=:id");
         $stmt2->execute(array('id' => $row['*field2'], 'plus' => $plus));
     }
 }
 ?>
 <form action="" method="post" name="Nyt" target="_self">
     <input name="plus" type="text"><br />
     <input name="Opdater" type="submit" value="Opdater">
 </form>