Below are two methods commonly used in most php codes for fetch mysql data .
mysql_fetch_array()mysql_fetch_assoc()
It depends on how your tables are setup:
mysql_fetch_array() essentially returns two arrays one with numeric index, one with associative string index.
So using mysql_fetch_array() without specifying MYSQL_ASSOC or MYSQL_NUM, or by specifying MYSQL_BOTH will return two arrays (basically what mysql_fetch_assoc() and mysql_fetch_row() would return) so mysql_fetch_assoc() is faster.
If you have your table setup right and query written properly mysql_fetch_assoc() is the way to go, code readability wise $result['username'] is easier to understand than $result[0].