PHP Mail() with Exim4 – PHP.ini Sendmail Path
if you install only exim4, you should get a symlink from /usr/sbin/sendmail to /usr/sbin/exim4. If you dont have it, you can create the symlink yourself.
ln -s /usr/sbin/sendmail /usr/sbin/exim4
Then no changes to your php.ini should be required.
ln -s /usr/sbin/sendmail /usr/sbin/exim ln: creating symbolic link `/usr/sbin/exim' to `/usr/sbin/sendmail': File exists
So, if you install exim after sendmail, your link should be created and you should not have to change php.ini