Dynamic drop down list not working - php

Question

I am trying to pass a js var to a php var, then use that php var to go through and array and populate a drop down list. I can see that the php var is getting the right value but list2 and list3 are not being populated.
If I put the string in myself $list1 = 'somestring'; it works, but when I use $list1 = $_POST['choice']; it doesn't work.
<script>
$(document).ready(function(){
$("#list1").change(function(){
var selected = this.value;
$.post("my.php", { choice: selected }, function(data){
alert(data);
});
});
});
</script>
<?php
echo'
<form name="menu">
<div>
<select name="list1" size="1" onchange="setOptions(document.menu.list1.options
[document.menu.list1.selectedIndex].value,document.menu.list2,document.menu.list3);">
<option value=" " selected></option>
<option value="one">item1</option>
<option value="two">item2</option>
<option value="three">item3</option>
<option value="four">item4</option>
</select><br><br>
<select name="list2" size="1"onchange="setOptions(document.menu.list2.options
[document.menu.list2.selectedIndex].value,document.menu.list3,' ');">
<option value=" " selected>Select an option</option>
</select><br><br>
<select name="list3" size="1">
<option value=" " selected>Select an option</option>
</select><br>
</div>
</form>';
?>
Drop down list
<script>
function setOptions(chosen, selbox, selbox2)
{
selbox.options.length = 0;
if (chosen == " ")
{
selbox.options[selbox.options.length] = new Option('Select an option',' ');
selbox2.options[selbox2.options.length] = new Option("Select an option"," ");
setTimeout(setOptions(' ',document.menu.list3),5);
}
if (chosen == "one")
{
showList2(selbox);
}
if (chosen == "two")
{
showList2(selbox);
}
if (chosen == "three")
{
showList2(selbox);
}
if (chosen == "four")
{
showList2(selbox);
}
//some code snipped
}
function showList2(selbox)
{
<?php
$n = 0;
$list1 = $_POST['choice'];
foreach($list[$list1] as $key => $value)
{
$key_array[$n] = $key;
$Lname[$n] = $list[$list1][$key]['name'];
$n++;
}
$jsArray = json_encode($key_array);
echo "var jsKeys = ". $jsArray . ";";
$jsArray = json_encode($Lname);
echo "var jsNames = ". $jsArray . ";";
?>
for (var i=0;i<jsKeys.length;i++)
{
var result = selbox2.options[selbox2.options.length] = new Option(jsNames[i],jsKeys[i]);
if(i == jsKeys.length-1)
{
var result = setTimeout(setOptions(jsKeys[0],document.menu.list3,' '),5);
}
}
return(result);
}
</script>

Answer 1

The $_POST superglobal is only set when the js function fires, ie when you choose something from the dropdown list. Otherwise remains empty.
You can check what's in the $_POST array with print_r($_POST)
Normally, I use a utility method to retrieve data from arrays:
function arrGet($array, $key, $default = NULL)
{
return isset($array[$key]) ? $array[$key] : $default;
}
This returns the value for the key or the default. Usage like:
if(arrGet($_POST, 'choice')){
//do something
}
Hope this helps...

Answer 2

<script>
$(document).ready(function(){
$("#list1").change(function(){
var selected = this.value;
$.post("my.php", { choice: selected }, function(data){
alert(data);
});
});
});
</script>
that little # in $("#list1") means the id of the element should be matched.
<select name="list1" ..... change it to <select name="list1" id="list1".....>