In core/error_api.php around line 166

function error_handler( $p_type, $p_error, $p_file, $p_line, $p_context ) {
}

                    # don't send the page header information if it has already been sent
                   if( $g_error_send_page_header || $g_error_send_page_header == null ) {
                            if( $t_html_api ) {
                                    html_page_top1();
                                    if( $p_error != ERROR_DB_QUERY_FAILED && $t_db_connected == true ) {

The condition check of $g_error_send_page_header always evaluates to true because any non 0, non negative number will trigger the first part of the if statement, and any 0, null, empty string, or negative number will trigger the second check.

I think the second check should use three equal signs when checking against null.