Vim has OmniCompletion built in, you should add this to your .vimrc:
filetype plugin on
au FileType php set omnifunc=phpcomplete#CompletePHP
In addition I recommend you this plugins:
- VTreeExplorer
- snippetsEmu
and also take a look to this article about php debugging in Vim, and this paper, it has many useful tips for using Vim in PHP Development.