MySQLi return NULL when there is no further entry in the table,
MySQL returns false in the same case.
I think that should be made consistent for all dbms.

http://www.php.net/manual/en/mysqli-result.fetch-row.php#38766

from "README.PHP4-TO-PHP5-THIN-CHANGES"

4. Be careful when porting from ext/mysql to ext/mysqli. The following
functions return NULL when no more data is available in the result set
(ext/mysql's functions return FALSE).

• mysqli_fetch_row()
• mysqli_fetch_array()
• mysqli_fetch_assoc()