Alright,
the correct answer is to use exit code 75. If your PHP script (or any other script) returns an exit code of 75, postfix will defer the message.
http://www.postfix.org/FILTER_README.html
10 # Exit codes from <sysexits.h>
11 EX_TEMPFAIL=75
12 EX_UNAVAILABLE=69
Jun 1 10:48:49 hostname postfix/local[32127]: C7F332C1: to=<test@domain.tld>, relay=local, delay=1, status=deferred (temporary failure)
Hope this helps the next person trying to find this information.