For Unix (tested on Gentoo, php 5.2.5)
You can also use
/usr/local/php5/bin/pecl install fileinfo
path to the pecl binary may depend on your system.
When installation is done, to autoload the module, in php.ini :
Note : check your extension dir if it fails.
You can check if the extension is correctly loaded with php -m
When installing fileinfo on php5.2 on ubuntu and having a problem with phpize,
you must also install the php5-dev package..
apt-get install php5-dev
Then verify if 'phpize' is in your PATH
for gentoo you will need to emerge pecl-fileinfo
on opensuse 10.2 systems you'll need the file-devel package.
For Kubuntu systems and probably most other Linuxes, after installing libmime-devel, I could successfully install the PECL Fileinfo package.
$ sudo apt-get install libmagic-dev
$ sudo pecl install Fileinfo