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The PHAR extension is missing in the Windows versions of PHP 5.3.0RC1 and snapshots.
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All users are prompted to test PHP 5.3 before the release. For Windows users, there's no easy way to test PHP 5.3 with Phar if it is not a part of the archive.
It is *builtin*.
Run: php -m
I see, sorry. I've updated the docs at least.