The post-install script class must contain two methods, one named init(),
and the other named run(). The init() method is called
at the same time as all other post-install scripts. The run() method is
called at the conclusion of each parameter group in order to process the user's responses
The package.xml contents as abstracted by this object.
The last version of this package that was installed. This is a very important
parameter, as it is the only way to determine whether a package is being installed
from scratch, or upgraded from a previous version. Using this parameter, it is
possible to determine what incremental changes, if any, need to be performed.
if $paramGroupId is _undoOnError, then
$infoArray will contain a list of successfully completed parameter group
sections. This can be used to restore any system changes made by the installation
Otherwise, $infoArray contains the results of the user input
from the most recent <paramgroup> section.
This variable either contains _undoOnError or the contents
of the most recent <paramgroup>'s <id> tag. Note that paramgroup
id cannot begin with an underscore (_), and so
_undoOnError can only be triggered by the PEAR installer.