This is what I'm doing:

$ php
<?php
$finfo=finfo_open(FILEINFO_MIME, '/usr/share/file/magic');

This is what I'm getting:

PHP Notice:  finfo_open(): Warning: description `8-bit ISDN mu-law compressed (CCITT G.721 ADPCM voice data enco' truncated in - on line 2
PHP Notice:  finfo_open(): Warning: description `8-bit ISDN mu-law compressed (CCITT G.721 ADPCM voice data enco' truncated in - on line 2
PHP Notice:  finfo_open(): Warning: <= not supported in - on line 2
PHP Notice:  finfo_open(): Warning: <= not supported in - on line 2
PHP Notice:  finfo_open(): Warning: <= not supported in - on line 2
PHP Notice:  finfo_open(): Warning: >= not supported in - on line 2
PHP Warning:  finfo_open(): Failed to load magic database at '/usr/share/file/magic'. in - on line 2

This is a clean CentOS 5.5 installation, PHP 5.3. The file /usr/share/file/magic exists and is accessible.