Description:
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I have 'implemented' MySQL's mysql_warning_count() function. (
http://dev.mysql.com/doc/refman/5.1/en/mysql-warning-count.html )

This function returns the number of warnings from the previous query. In some queries (INSERT INTO .. VALUES (..), (..); INSERT INTO .. SELECT; etc) mysql_info() was usable to return the number of warnings; but for single-row inserts mysql_info() returns false. 

The function is very straightforward, almost the same as mysql_thread_id().
mysql_warning_count() is available in MySQL's C-api in >3.23, >4.1 and >5

The patch is available at http://jille.hexon.cx/mysql-warning-count.diff
It is based on PHP 5.3.0 and at least works for me.