populating different data with select onChange in php

i need some clarification on how to populate select(s) with data from mysql. Basically what I am trying to do is:

There will be a first select box with some data in it.

<select> 
  <option>1</option>
  <option>2</option>
  <option>3</option>
</select>

when the user selects a option in开发者_如何学JAVA the first select, there is a second select below that, which should reflect the values according to the selection made in the first select.

<select> 
  <option>1.1</option>
  <option>1.2</option>
  <option>1.3</option>
</select>

The data is commin from MySQL. I am not sure if need to post to same page, but if I do, how to retain the values alredy selected in the previous select boxes? do i need to use javascript?

any help?

Thanks.

---

You should use javascript so you don't need a page refresh. I just re-read your question and I'll have a solution involving an AJAX request in a second to pull dynamic data:

HTML

<select name="select1" id="select1"> 
  <option value="1">1</option>
  <option value="2">2</option>
  <option value="3">3</option>
</select>

<select name="select2" id="select2"> 
  <option value="1">1</option>
  <option value="2">2</option>
  <option value="3">3</option>
</select>

jQuery

<script type="text/javascript">
$(document).ready(function() {
    $('#select1').change(getDropdownOptions);
});

function getDropdownOptions() {
    var val = $(this).val();
    // fire a POST request to populate.php
    $.post('populate.php', { value : val }, populateDropdown, 'html');
}

function populateDropdown(data) {
    if (data != 'error') {
        $('#select2').html(data);
    }
}
</script>

populate.php

<?php
if (!empty($_POST['value'])) {
    // query for options based on value
    $sql = 'SELECT * FROM table WHERE value = ' . mysql_real_escape_string($_POST['value']);

    // iterate over your results and create HTML output here
    ....

    // return HTML option output
    $html = '<option value="1">1</option>';
    $html .= '<option value="b">B</option';
    die($html);
}
die('error');
?>