hey, ok now lets hope i can get this across in as simple a way as possible !

ok, i want to make an encryption. text is inputted in a form. is then displayed the other side encrypted. I've set all that up. and it works. But i want to make it a bit different. ok heres an example:   

say i = 5 and t = 35     ...   at the moment it would come out as 5/35   ... how could i change it so that say it took away the amount of which it is in posision...

for instance i is first, so it would be 5-1... and t is second so it would b 35 - 2... so the new encryption would be 4/33  ...  any ideas how i would do this please?


ah yes, forgot to mention, i'm doing this in php