[PHP-DB]how to handle exception in php

hi friends

I am doing a simple student mark details project.
for calculating rank I need to compare the total marks one by one.
To do so i use *for loop.* so atlast while reaching the end of the table it
shows the following warning.
*
Warning*: mysql_result()
[function.mysql-result<http://127.0.0.1/pro/function.mysql-result>]:
Unable to jump to row 18 on MySQL result index on line *50*

It is because it has no next value to compare.How to use try..catch
exception handling in this.?Or any other suggestion.
Can anybody help me

--Bavithra