1. This version of PHP seems unwilling/unable to raise the post_max_size variable. Various, repeated attempts show that it is "stuck" at 8MB. The upload_max_filesize paramater does not have this problem. While this isn't a problem with Zabbix, the fix is to change include/requirements.inc.php so that check_php_post_max_size() allows $required to 8 MB.
2. the db.inc.php snippet fails at getSchema()... include(self::SCHEMA_FILE) unless the php_include_path is set to include the zabbix/include directory. This is not documented anywhere. This can be in the INI file, but for safety, it is better to put this in either a .htaccess file or as a virtual host configuration directive in the HTTP configuration:
php_value include_path ".:/usr/share/zabbix/include:/usr/share/pear"