This is a prototype based on arnaud-lib/vim-php-namespace, but uses CtrlP to select from the list of matches. It intentionally doesn't do expansion, as I don't really do that anymore.
inoremap <buffer> <leader>u <C-O>:let g:backToInsert=1<CR><C-O>:call PhpInsertUse()<CR><C-O>a
noremap <buffer> <leader>u :call PhpInsertUse()<CR>