More efficient hierarchy system - php

Question

I am trying to code a hierarchy detection script, I've already written the script, but can only go down 4 levels. Is there a way to condense this to a few lines that will work with an infinite amount of levels?
This script is basically the same code copy and pasted 4 times
<?php
function listCategories($name, $disable_status = 0, $show_nums = 0) {
echo "<select name='".$name."'>";
$result = mysql_query("SELECT * FROM categories") or die(mysql_error());
while($row = mysql_fetch_array($result)) {
if($row['parent_id']==0) {
$result2 = mysql_query("SELECT * FROM categories WHERE parent_id=".$row['id']) or die(mysql_error());
echo "<option value='".$row['id']."'";
if($disable_status==1&&isParent($row['id'])){
echo " disabled='disabled'";
}
echo ">".$row['name']."</option>";
while($row2 = mysql_fetch_array($result2)) {
$result3 = mysql_query("SELECT * FROM categories WHERE parent_id=".$row2['id']) or die(mysql_error());
echo "<option value='".$row2['id']."'";
if($disable_status==1&&isParent($row2['id'])){
echo " disabled='disabled'";
}
echo ">- ".$row2['name']."</option>";
while($row3 = mysql_fetch_array($result3)) {
$result4 = mysql_query("SELECT * FROM categories WHERE parent_id=".$row3['id']) or die(mysql_error());
echo "<option value='".$row3['id']."'";
if($disable_status==1&&isParent($row3['id'])){
echo " disabled='disabled'";
}
echo ">-- ".$row3['name']."</option>";
while($row4 = mysql_fetch_array($result4)) {
echo "<option value='".$row4['id']."'>[".$row4['id']."] --- ".$row4['name']."</option>";
}
}
}
}
}
echo "</select>";
}
function isParent($cat_ID) {
$result = mysql_query("SELECT * FROM categories WHERE parent_id=".$cat_ID) or die(mysql_error());
if(mysql_num_rows($result)==0) {
return FALSE;
} else {
return TRUE;
}
}
My table structure for categories is
id, name, parent_id
If the category has no parent, the parent_id will be 0, or else, it would be the id of the category of it's parent.
All help is appreciated.

Answer 1

I guess something along these lines should do (untested, must be adapted to your needs):
$q = mysql_query("SELECT id, parent_id, name FROM categories");
while ($r = mysql_fetch_row($q)) {
$names[$r[0]] = $r[2];
$children[$r[0]][] = $r[1];
}
function render_select($root=0, $level=-1) {
global $names, $children;
if ($root != 0)
echo '<option>' . strrep(' ', $level) . $names[$root] . '</option>';
foreach ($children[$root] as $child)
render_select($child, $level+1);
}
echo '<select>';
render_select();
echo '</select>';
an even funkier way of doing this is by using SQL stored procedures, but it may be way overkill in this case...

Answer 2

This isn't an answer but instead of using many selects you can use many left joins to traverse 4 hierarchies. Instead of so many row of codes it's just this single query:
select a.id,a.parent_id,b.id,b.parent_id,c.id,c.parent_id,d.id,d.parent_id from a left join b ON a.id = b.parent_id left join c on b.id=c.parent_id left join d on c.id=d.parent_id;
You can use my query with a single table, too, because you can alias a table name:
left join mytable as c on c.id=d.parent_id ... ...
But unfortunetley mysql doesn't support recursive selects. Maybe try another database?