php_pear_channel "pear.symfony-project.com" do
...fails because in the exists, "channel-info" is used and a Mixlib::ShellOut::ShellCommandFailed is thrown in Chef 0.10.10.
rescue Chef::Exceptions::ShellCommandFailed is required by 0.10.8
rescue Mixlib::ShellOut::ShellCommandFailed is new in 0.10.10
rescue NameError handles the missing Mixlib::ShellOut::ShellCommandFailed for 0.10.8