Complex grouping

Hi,

I am developing airplane tickets reservation system. Let's say I have an array of the roundtrip flights. Each flights has its price, outbound flight id and inbound flight id. For now let's assume there are only four roundtrip flights there:

 $roundtrips = array(
    array('price'=>'100', 'out'=>'10', 'in'=>'50'), 
    array('price'=>'100', 'out'=>'10', 'in'=>'60'),
    array('price'=>'200', 'out'=>'20', 'in'=>'70'),
    array('price'=>'200', 'out'=>'30', 'in'=>'60'),
);

PHP:

I need to group this roundtrips by price to have output like this:

<table border="1" width="300">
<tr>
<td colspan="2"><h3>$100</h3></td>
</tr>
<tr><td colspan="2"><strong>Flights out</strong></td></tr>
<tr><td width="270"> &nbsp;Flight 10</td><td width="30" align="center"><input type="radio" name="flight_out" value="10" /></td></tr>
<tr><td colspan="2"><strong>Flights in</strong></td></tr>
<tr><td width="270"> &nbsp;Flight 50</td><td width="30" align="center"><input type="radio" name="flight_in" value="50" /></td></tr>
<tr><td width="270"> &nbsp;Flight 60</td><td width="30" align="center"><input type="radio" name="flight_in" value="60" /></td></tr>
<tr><td colspan="2" align="right"><input type="submit" /></td></tr>
<tr>
<td colspan="2"><h3>$200</h3></td>
</tr>
<tr><td colspan="2"><strong>Flights out</strong></td></tr>
<tr><td width="270"> &nbsp;Flight 20</td><td width="30" align="center"><input type="radio" name="flight_out" value="20" /></td></tr>
<tr><td width="270"> &nbsp;Flight 30</td><td width="30" align="center"><input type="radio" name="flight_out" value="30" /></td></tr>
<tr><td colspan="2"><strong>Flights in</strong></td></tr>
<tr><td width="270"> &nbsp;Flight 60</td><td width="30" align="center"><input type="radio" name="flight_in" value="25" /></td></tr>
<tr><td width="270"> &nbsp;Flight 70</td><td width="30" align="center"><input type="radio" name="flight_in" value="25" /></td></tr>
<tr><td colspan="2" align="right"><input type="submit" /></td></tr>
</table>

Code (markup):

And I can do it! However, look closely to the group of $200 price. The user can select the flowing combinations of outbound and inbound flights: 20/60, 20/70, 30/60, 30/70. But the combinations 20/60 and 30/70 are not valid, because there are no roundtrips with such 'out' and 'in' flights!

So, in this case there should be 2 groups priced $200 so that the user couldn't select invalid combinations.

How do I do this kind of grouping? Please help, I'll appreciate it.