The script will check some php.ini settings warn you if they
are set incorrectly, and then download the latest composer.phar
in the current directory. These lines will, in order:

- Download the installer SHA-384 to the current directory
- Download the installer to the current directory
- Verify the installer SHA-384 which you can also cross-check https://composer.github.io/pubkeys.html
- Run the installer
- Remove the installer & the installer SHA-384
Signed-off-by: Rémy Coutable <remy@rymai.me>