I am working on a php/c# project which is an Email Server with a web interface to manage the Email Server application.
The program is supposed to work on windows an
You standard php mail function will just send to whatever is defined as the sendmail _path in php.ini
This is typically sendmail -t -i
You would need to configure sendmail to use smtp.
FWIW, most developer who do a lot of mail sending from PHP apps revile the mail() and instead use one of many mailing libraries (or services) which provide better configurability/reliability.
You could for example pipe the mail function to your own PHP script and use whatever library you wanted to in that script in order to do mail sending (and thus preserving the use of mail() function across applications).