Autoloading PEAR packages with composer.

composer.json

{
    "require": {
        "pear-pear/pear": "1.9.4",
        "pear-pear/Mail_Mime": "1.8.7"
    },
    "repositories": [
        {
            "type": "pear",
            "url": "http://pear.php.net"
        }
    ]
}

test.php

<?php

    require 'vendor/autoload.php';
    var_dump(class_exists('Mail_mime') && class_exists('Mail_mimePart'));

$ composer install
Loading composer repositories with package information
Initializing PEAR repository http://pear.php.net
Installing dependencies
  - Installing pear-pear.php.net/xml_util (1.2.1)
    Downloading: 100%         
  - Installing pear-pear.php.net/console_getopt (1.3.1)
    Downloading: 100%         
  - Installing pear-pear.php.net/structures_graph (1.0.4)
    Downloading: 100%         
  - Installing pear-pear.php.net/archive_tar (1.3.11)
    Downloading: 100%         
  - Installing pear-pear.php.net/pear (1.9.4)
    Downloading: 100%         
  - Installing pear-pear.php.net/mail_mime (1.8.7)
    Downloading: 100%         
Writing lock file
Generating autoload files

$ php test.php 
bool(true)