M-x customize-variable RET backward-delete-char-untabify-method RET

(RET = Enter in Emacs speak.)

Also, you don’t need to do “(require ‘php-mode)” when you define and set a hook function. It loads php-mode file but it’s not necessary at Emacs load time; it just slows down your Emacs startup. The hook function will be executed after you enter the php-mode.

Yes, with open-ended systems like Emacs there is always much to learn. It’s never “ready” because it’s so open for user’s customization.