It depends on what sender address you are talking about. The sender address visble in the recipients mailprogramm is extracted from the "From:" Header. which can probably easily be set from your program.
If you are talking about the SMTP envelope sender address, you can pass the -f argument to the sendmail binary. Depending on the server configuration you may not be allowed to do that with the apache user.
from the sendmail manpage :
-f <address>
This option sets the address of the envelope sender of a
locally-generated message (also known as the return path).
The option can normally be used only by a trusted user, but
untrusted_set_sender can be set to allow untrusted users to
use it. [...]