从Pear安装类到PHP（Ubuntu）

I just wanted to install this class from Pear:

http://pear.php.net/package/Mail_mimeDecode/

After this command:

sudo pear install Mail_mimeDecode-1.5.5

I have this:

downloading Mail_mimeDecode-1.5.5.tgz ...
Starting to download Mail_mimeDecode-1.5.5.tgz (11,554 bytes)
.....done: 11,554 bytes
downloading Mail_Mime-1.8.9.tgz ...
Starting to download Mail_Mime-1.8.9.tgz (33,796 bytes)
...done: 33,796 bytes
install ok: channel://pear.php.net/Mail_Mime-1.8.9
install ok: channel://pear.php.net/Mail_mimeDecode-1.5.5

So everything looks fine. But when i want to use this class i got this error:

Class 'Mail_mimeDecode' not found

I try use it like this ($xml is a string with an XML inside):

$mime = new \Mail_mimeDecode($xml);

I also don't see it in my phpinfo Profiler. Am I missing something ? I work on Symfony2 Application if that changes anything...