remove exim4 and install postfix ..or
ln -s /usr/sbin/sendmail /usr/sbin/exim4
dpkg-reconfigure exim4-config which should create a symlink of sendmail to the exim binary.
You shouldn't find out...you should know
Debian defaults to using exim4 as mail server.
In php.ini there is a line that says
sendmail_path = /usr/sbin/sendmail -t -i