Upon further searching this sounds like bug report 25195 
which had no solution given.  Any chance someone has 
discovered a work-around for this?

This problem is well known and a problem of Solaris in all versions. For PHP 4.3.4 we ported a lot of code from PHP5 back to eliminate fopen(),... Scripts now start without any problems (until PHP 4.3.3 even this could fail). But there are still a lot of extensions that use stdio functions.

A solution would be to write a temporary file with the complete mail (headers and text) and then execute: exec("/bin/sh -c \"/usr/lib/sendmail -t </tmp/mail.txt\"").

Thanks for the quick feedback.  However, we have a similar problem with exec so the suggested work-around won't help.

I'm going to try to rewrite mail.c to use sfio from AT&T.  The downside is that I'll then have to reimplement that fix everytime we upgrade php.  I can share the code I come up with if anyone is interested.  If it does work, then I'll try to look more into the exec function as well.

If it doesn't work then another work-around I thought of is to have php write the email to a file and then have a cron-scheduled system script pass the file off to sendmail.

If you continue to have this problem, you may want to look into a package which implements mailing in PHP code, such as:

http://pear.php.net/package/Mail

Here's a solution that appears to work for us so far.  It might be nice if the php developers would consider a --with-sfio parameter in configure that took care of this.  Our exec command is still broken so I'll probably have to do this same type of thing for that.

1.  install sfio from AT*T
2.  setenv LIBS "-L/usr/local/lib/sfio -lsfio"
3.  modify ext/standard/mail.c
4.  modify /usr/local/include/sfio/sfio.h so references to other sfio header files are absolute paths (if I use -I/usr/local/include/sfio to find the files it produces errors for files other than mail.c)
5.  run configure and compile as normal

here's a diff or my code and the original mail.c:

diff mail.c mail.c.orig
23c23
< #include </usr/local/include/sfio/sfio.h>
---
> #include <stdio.h>
177c177
<         Sfio_t *sendmail=NULL;
---
>       FILE *sendmail;
215c215
<         sendmail = sfpopen(sendmail, sendmail_cmd, "w");
---
>       sendmail = popen(sendmail_cmd, "w");
224c224
<                       sfclose(sendmail);
---
>                       pclose(sendmail);
228,229c228,229
<               sfprintf(sendmail, "To: %s\n", to);
<               sfprintf(sendmail, "Subject: %s\n", subject);
---
>               fprintf(sendmail, "To: %s\n", to);
>               fprintf(sendmail, "Subject: %s\n", subject);
231c231
<                       sfprintf(sendmail, "%s\n", headers);
---
>                       fprintf(sendmail, "%s\n", headers);
233,234c233,234
<               sfprintf(sendmail, "\n%s\n", message);
<               ret = sfclose(sendmail);
---
>               fprintf(sendmail, "\n%s\n", message);
>               ret = pclose(sendmail);