I'm getting the following error when I try to use the PHP mail() function.
Call to undefined function: mail()

According to the research I've done, for whatever reason when PHP was built, it could not find the sendmail binary...so I need to rebuild/recompile PHP with the sendmail binary installed.

FWIW, the sendmail binary is in what I understand is the usual place:
/usr/sbin/sendmail

Problem is, I've no clue where the source code is for PHP, and even if I could find it, I know very little about building from source or even installing RPMs...I'm a complete Linux noob. Or maybe it's done via Yast?

The server is running SuSE 9.1
PHP version is 4.3.4
Server is Apache 2.0.4.9

Any help would be greatly GREATLY appreciated.

You can get the PHP source from php.net here. If you want the older (and now unsupported) versions, go here. I'm not sure about YaST, as I don't use SuSe. There is a guide to compiling from source here. One thing you might want to do is make a PHP page with

in it, so you can see the what the "Configure Command" is (it's near the top of the page in the output from phpinfo(). I'm only suggesting this because it might be a good idea to use the same options when you do ./configure when building PHP from source (read the guide I gave you, it'll clear that up).

Hope that helps.

Recently I had to use that funtion over SuSE 9.1 and during the way to the solution solution I step in here. The only problem it gots is that the Postfix server do not allow you to send emails to the outside world. If postfix is running and you post a mail using the php function it will work just fine bu you must enable the other interfaces at post fix.
At /etc/postfix/main.cf look for "inet_interfaces = 192.168.X.X 127.0.0.1 ::1" where 192.168.X.X is the server local ip. Then reload / restart the service. That is all I had to do and it worked out. That's all folks! Good luck! cU!
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Ezequiel