How do I install / enable the PHP phar extension?

I am trying to install Composer on my KnownHost VPS. When I run this command:

curl -sS https://getcomposer.org/installer | php

I get this error message:

Some settings on your machine make Composer unable to work properly.
Make sure that you fix the issues listed below and run this script again:

The phar extension is missing.
Install it or recompile php without --disable-phar

How do I install the phar extension? I am running PHP 5.4.22 on my VPS.

Answer

In the end I solved this by getting my host to rebuild PHP with PDO support.