There are a few ways to achieve that, here is for a single file:
This way is a bit more automated:
vi --run-command=':set bomb, :set fileencoding=utf-8' filename.php"
And here is a way for various files:
vim +"set bomb | set fileencoding=utf-8 | wq" $(find . -type f -name *.php)
that will also work:
find . -type f -name *.php | xargs vim +"argdo set bomb | set fileencoding=utf-8 | w"