Postfix not delivering mails

I'm not quite sure what you are trying to do here. I'd suggest reading the CONTENT_FILTER_README and SMTPD_POLICY_README in the Postfix documentation. Your test.php script should probably be running as a content filter rather than a delivery-time script. Especially if you are using the script to reject messages based on sender/recipient, it's considered bad form to make those sorts of decisions after you've accepted the message for delivery.

The usual way to do content filtering in Postfix is for the front-end smtpd to hand off to a content filter via SMTP, LMTP or a pipe, and the content filter to reinject into an entry point that has no content filtering enabled -- either another smtpd with the content_filter parameter disabled in a -o override in master.cf, or local submission using /usr/sbin/sendmail with the Postfix pickup daemon's content_filter disabled in master.cf.

you can't have parallel delivery with mbox format.

 local_destination_recipient_limit = 1   #this is default, just remove from main.cf
 local_destination_concurrency_limit = 1

If the lock problem persist, then you have filesystem error or hard drive problem.