install emacs on debian

aptitude install emacs-common
aptitude install emacs21

search for module to emable php-mode:
apt-cache search emacs | grep php
php-elisp - Emacs support for php files

apt-get install php-elisp

W: There are no public key available for the following key IDs

If you just ran apt-get update and you got:

There are no public key available for the following key IDs:
A70DAF536070D3A1

then type:

sudo gpg --recv-key --keyserver wwwkeys.eu.pgp.net A70DAF536070D3A1
sudo gpg --export A70DAF536070D3A1 | apt-key add -

and all will be well.