WebStar versions 5.2.4 and greater have no PHP problem with SquirrelMail. Below is for historical purposes only.
WebStar V (v5.1.2) for Mac OS X ("WSV") comes with a precompiled PHP (based on PHP 4.1.2, I believe). There is both a CGI and Fast CGI version included. You will have to edit the "php.ini" file (there are separate copies in WebServer/cgi-bin and WebServer/fcgi-bin) to enable "register_globals", as it comes set to "Off" by default.
However, there seems to be a problem with WebStar's PHP implementation which affects SquirrelMail. In a number of places in its code, SquirrelMail redirects to a different page by using the "header()" command to write a "Location:" HTTP header. According to some PHP docs that I found (http://www.php.net/manual/en/function.header.php), this should also set the HTTP status code returned to the client to 302, for redirection. However, WebStar doesn't check for this, and returns "HTTP/1.1 200 OK", so most clients won't do the redirection even though the "Location:" header is there also. Presumably this works properly in the PHP environments SquirrelMail runs in. I've submitted it to the WebStar bugbase.
I've also added an extra line of code wherever a redirection is done, which gets around this. All you need do is something like the following in the various SquirrelMail source code files:
header("Location:..."); // original line in !SquirrelMail
header("HTTP/1.1 302 Redirection"); // explicitly setting the HTTP status line gets around !WebStar bug
After doing this, I was able to successfully use SquirrelMail to log into an account on our IMAP server, and send an email. Further testing will have to wait for some more free time.
[July 3/2002] - I've posted these modifications at: http://www.stats.gov.nt.ca/oberst/Tech/SquirrelMail.html