You want the IN clause.
select whatever from some_table where the_id in ( 1, 2, 5, 89094037 ) order by whatever;
example:
page1.php
<form action="page2.php" method="post">
<input type="checkbox" name="check_me" value="12345"> 12345
<input type="checkbox" name="check_me_2" value="67890"> 67890
<input type="submit" value=" Submit ">
</form>
page2.php
<?
$sql = "select * from check_items where check_item_id in (" . $POST['check_me'] . ", " . $POST['check_me_2'] . ")";
query as usual
?>