You can get the result of the form when submitted and use the id from there, no need to add anything to the database and call it back if it isn't necessary for what you're trying to accomplish:
PHP Code:
$somevariable = $mysidia->input->post("petlist");
(Following the example code above)
Now you can use '$somevariable' to get the id of the chosen pet and use it elsewhere in the script; like so:
PHP Code:
if($mysidia->input->post("submit")){
$petid = $mysidia->input->post("petlist");
/*javascript goes here*/
}
This is in a case where it gets the id if the user clicks a submit button. If you wanted to do something that updates without reloading/changing pages then I doubt you would use this exact code, but you can still get the id the same way.