How to make a dropdown list from categorized data in php

How to make a dropdown list from categorized data in php

Have a Data like
Id , Title, parent_id
'1', 'Operating System Issues', '1',
'2', 'Hardware Issues', '2',
'3', 'Window\'s Didn\'t Boot',
'4', 'Boot Error', '1',
'5', 'No Login', '1',
'6', 'Loud Noise', '2',
'7', 'No Display', '2',
want it to represent like following in drop down list: -
Operating System Issues
Boot error
No Login
Windows didnt boot
..
..
Hardware Issues
Loud noise
No Dispaly
..
..
and so on.
This sql query select group_concat(title) as titles, parent_id from
quality_failures group by parent_id; makes my drop down looks like :
Select
Operating System Issues,Window's Didn't Boot,Boot Error,No Login
Hardware Issues,Loud Noise,No Display
How can I make it look like as described above.