<!-- change "zzzz" to the url of the code, like "index.php" -->
<form action="zzzzz" name="mySelectForm">
<!-- this is the select-box with some additional javascript, to have it submit
the form, whenever you make changes to the value -> NO NEED 2 CHANGE -->
<select name="mySelect" onChange="document.mySelectForm.submit();">
<!-- the php part changes the default-values to the one you choose before -->
<!-- here are 2 options,
"XXX" is the name( capital letters )
"xxx" its value, CHANGE THAT TO FIT YOUR PROBLEM(13items) -->
<option value="xxx"<?php if ( $_REQUEST[ 'mySelect' ] == 'xxx' ){ echo ' selected'; ?>>XXX</option>
<option value="aaa"<?php if ( $_REQUEST[ 'mySelect' ] == 'aaa' ){ echo ' selected'; ?>>AAA</option>
</select>
</form>
<?php
// print your stuff here,
// the selected value is in $_REQUEST[ 'mySelect' ]
?>