We provide several PHP Mode installation methods.

Install via package.el

Modern Emacs extensions (often called Lisp package) can be installed from ELPA (GNU Emacs Lisp Package Archive).

However, PHP Mode is not registered in default repository (GNU ELPA) due to historical reason.

Add MELPA to package-archives

If you have not already registered MELPA in package-archives, read Getting Started - MELPA and add MELPA.

M-x list-packages

Press M-x list-package and search php-mode package. If you find php-mode, typing I followed by x will install the package.