Thread safety

[Ravi Kumar.]

Dear All:
 
I have a table T1 with these columns:
 
    location - varchar
    odd_even - varchar
    
I have a php page that is called from VC applications installed in 5
different locations (L1, L2, L3, L4 and L5). The page returns 0 or 1
depending on even call or odd call from each location. (1st, 3rd, 5th,
7th... page calls are odd and 2, 4, 6, 8 are even.)
 
The page has this logic:
 
    select odd_even from T1 where location = '$location'
 
    if (odd_even == 'odd')
        update T1 set odd_even = 'even' where location = '$location' and
odd_even = 'odd';
        return '0';
    else 
        update T1 set odd_even = 'odd' where location = '$location' and
odd_even = 'even'; 
        return '1';
 
But since each location calls the page in very quick succession (a location
have have several parallel calls running - its a multi threaded app), there
is a race condition. And at times several continuous calls return same
result (odd or even).
 
How do I achieve thread safety?
 
Please help. Its urgent.
 
Thanks.
 
Ravi.