This code in .emacs did the job:
(add-hook 'php-mode-hook (lambda () (make-local-variable 'c-basic-offset) (setq c-basic-offset 2)))
(add-hook 'php-mode-hook
(lambda ()
(make-local-variable 'c-basic-offset)
(setq c-basic-offset 2)))
I failed previously because I tried to change the global c-basic-offsetc-basic-offset
. That didn't help as the local variable generated in php-mode took precedence (I think).