Currently the installation manual http://php-mode.sourceforge.net/html-manual/Installation.html proposes to load the PHP mode file in any case:
However, even an ambitious programmer does not need the mode each time. Therefore, I propose to rather recommend in the manual the following usual method:
(autoload 'php-mode "php-mode" "PHP edit major mode" t)
If each available package was loaded by the Emacs, the start-up time would last unacceptably long.