I gave you some links, in your other question, which you should really take a look at.
As this question is more specific: below is an example form, simplified for what you have currently (as it is 'just' an example), which you should be able to figure out what to do, as everything you need will be in it (apart from the database side).
shop.php
<!-- Notice that the method used is "post", this hides the data submited in the background. -->
<!-- "get" on the otherhand, shows the values submited in the url. -->
<form action="submitpage.php" method="post">
<!--
<select name="choice">
<?php
$available = 4;
for ( $i = 0; $i <= 4; $i++ )
{
?>
<option value="<?php echo $i; ?>"><?php echo $i; ?></option>
<?php
}
?>
</select>
<input type="submit" value="Submit" />
</form>
Below is the page which the form is directed to. It outputs the value of the select box.
submitpage.php
<?php
$choice = isset( $_POST[ 'choice' ] ) ? $_POST[ 'choice' ] : 0 ;
echo 'Choice = ' . $choice;
echo '<br /><br />';
echo '<pre>'.print_r( $_POST , true ).'<pre>';
?>
Read the comments for more explanation.