How to write a query that allows multiple input from a select list

[Max58kl]

I have a select list that allows me to filter the results of a database search.
The user chooses from a dropdown menu then clicks submit.
The target page shows only the records containing what was chosen from the list.
What I would like is to allow the user to choose multiple items from the
list while holding down the ctrl key, then filter the search results based on what was chosen.

Can you tell me how to change the syntax on my target page query so that is allows multiple input.

something like -

WHERE titles.level_id IN (1,2,3,4)

This is the current query

$query_rsTitles = "SELECT id, title, company, `description`, resources, location, url, image, keyword, copies FROM titles WHERE titles.level_id = ". GetSQLValueString($_POST['softwareLevel'], "int") ." ORDER BY id ASC";

This is the menu code -

<select name="softwareLevel[]" id="softwareLevel">
          <?php
  do {  
  ?>
          <option value="<?php echo $row_rs_level['level_id']?>"><?php echo $row_rs_level['level']?></option>
        <?php
  } while ($row_rs_level = mysql_fetch_assoc($rs_level));
    $rows = mysql_num_rows($rs_level);
   if($rows > 0) {
      mysql_data_seek($rs_level, 0);
      $row_rs_level = mysql_fetch_assoc($rs_level);
   }
 ?>
  </select>