Unlimited Category Tree - Need Help

[roosevelt]

For a while I've been trying to make this system and so far it goes up to one level. I am pretty sure there's a way to display unlimited category levels but that's what I just don't know. Anyways I will not post the actual source code but the concept should be enough for a more advanced PHP programmer.

Table
- CatId
- CatName
- CatParent

Example
CatId CatName CatParent (or subof)
1 Man 0
2 Woman 0
3 Boy 1
4 Girl 2
5 Baby Girl 4
6 Baby Boy 3

PHP Code:

$sql = mysql_query("SELECT * FROM categorytable WHERE CatParent = '0'");
while ($data = mysql_fetch_array($sql))
{
 echo "{$data['CatName']}<br />";
 $sql2 = mysql_query("SELECT * FROM categorytable WHERE CatParent = '".$data['CatId']."'");
 echo "- {$data2['CatName']}<br />";
} 


My little while loop produces this:
Man
- Boy
Woman
- Girl

So if I wish to display Baby Girl category under Girl I will need another loop just under $sql2. Now the problem is I don't really know how deep the user will create his categories. So what is the trick to make unlimited categories?

Thanks a bunch in advance!!