Undefined error: 0 OR Unable to load autoload.php. Drush now requires Composer
Drush switched to requiring composer. The documentation provides details on how to install both composer and drush, but it boils down to:


Install composer globally:

curl -sS https://getcomposer.org/installer | php
mv composer.phar /usr/local/bin/composer

Install drush globally (for your user), you can specify the version:

composer global require drush/drush:6.*

Add the composer path to your path (in your .bashrc or .zshrc or other):

export PATH="$HOME/.composer/vendor/bin:$PATH"