This is primarily a PHP/HTML question, so one of those forums would possibly provide a more authoritative answer, but...
Option-selects are a feature of forms.
Forms usually include a 'method' (GET or POST), an 'action' (usually the page that the form parser resides upon), and a 'Submit' button.
So, if using the GET method, your form might look something like this:
<form method='get' action='my_form_parser.php' name='form1'>
<select name='my_option'>
<option>1</option>
<option>2</option>
<option>3</option>
</select>
<input type='submit' name='submit' value='submit'>
</form>
and your parser might begin with a statement like:
<?php
$my_option = $_GET['my_option'];
$query = "SELECT details FROM my_table WHERE account_no = $my_option";
You may also want to look at AJAX methods.