making forum tree

i have mysql table like this:

id | r_id | level | porder

where

id - post id
r_id - parent post id
level - post position in the tree (horizontal)
porder - post position in the tree (vertical)

i had to write a php script which paint tree like this:

level porder
p 0 0
|-p 1 1
|-p 1 2
| |-p 2 3
| |-p 2 4
| | |-p 3 5
| | |-p 3 6
| | \-p 3 7
| | \-p 4 8
| \-p 2 9
| |-p 3 10
| \-p 3 11
\-p 1 12

obwiously i replace asci charts equivalent gif's.
'|-', '| ', '\-', ' ' are separate gif's.
i'll be very grateful if somebody help my write this script.