Problems following PHP upgrade

[Michael Doyle]

I am having problems following upgrading a specific port, and I would  
like advice on how to proceed
in diagnosing the fault.

The application showing symptoms of unreliable behaviour is  
"WebCalendar 1.2.0b1" (a php application)
http://www.k5n.us/webcalendar.php?topic=About

FreeBSD 6.4 stable -> Apache 2.2.11 -> PHP 5.2.11 & PostgreSQL 8.3.7 - 
 > WebCalendar 1.2.0

Server: FreeBSD 6.4 stable (compiled/installed Friday of last week).
Everything appeared fine after the OS update.

Apache 2.2.11 (Updated on Friday of last week. update caused no  
problems)

PostgreSQL 8.3.7 (Updated on Monday. Drupal and Webcalendar worked  
after update.

Took a little effort/time to get WikiMedia to update cleanly due to  
changes between PostgreSQL 8.2.x and 8.3.x
as a previously optional module had been moved in to the "core" of  
PostgreSQL).


Today I updated the installed version of PHP from PHP 5.2.6 to version  
5.2.10
At first all appeared to work, but then I noticed that certain pages  
within the WebCalendar application were failing to load.

/var/log/messages contains multiple lines of the form:
Jul 16 16:13:33 intranet kernel: pid xxx (httpd), uid 80: exited on  
signal 4

/usr/local/www/apache22/logs/httpd-error.log contains multiple lines  
of the form:
[Thu Jul 16 16:13:33 2009] [notice] child pid xxx exit signal Illegal  
instruction (4)

I don't see any apache core files dropped, but I might be looking in  
the wrong place.

I have re-installed PHP multiple times, including deleting and re- 
installing all the libraries that PHP depends on,
and re-booting the server to ensure that all the old versions of  
libraries are not lingering in memory.

Can someone advise me on where to look next and/or how to get further  
debugging information out of this
stack of applications