Assuming you're using vim-plug
" Include Phpactor
Plug 'phpactor/phpactor' , {'do': 'composer install', 'for': 'php'}
" Require ncm2 and this plugin
Plug 'ncm2/ncm2'
Plug 'roxma/nvim-yarp'
Plug 'phpactor/ncm2-phpactor'
Additionally you will need to set the following options:
autocmd BufEnter * call ncm2#enable_for_buffer()
set completeopt=noinsert,menuone,noselect