If you see the above error, you're probably running a php version not compatible with drush.
To remedy this, use the following command. This is on centos, on ubuntu or other linux flavors the path may be different.
First find the folder where the drush include file is located. You use the linux command
$find / -name "folder_you're_looking for"
Output should be something similar to this
find / -name "drush"
Use nano or other editors to go to
$ nano +1039 /usr/share/pear/drush/includes/bootstrap.inc
+1039 is the exact location yours could be different.
Change the line to something like this
Restart your apache
$service httpd restart
and error should be gone.