You can use recode
recode UTF-8 *.php -v
provided that all the files have the same encoding.
Otherwise you can try with something like
iconv -f US-ASCII -t UTF-8 *.php
a minor problem
It could be an apparent problem if there aren’t non-ascii characters in a file: then these files are yet recognized, with
file -i --mime-encoding *
as us-ascii.
So you can add a non ascii character, like è:
sed -i -e “$aè” *.php
and afterwards redo the command “recode” as above and replace some ascii expression, like agrave; with UFT-8 à character.
Eventually you will delete the “è” from the end of the files.