PEAR_ErrorStack::pushCallback() -- Set an error Callback If set to a valid callback, this will be called
every time an error is pushed onto the stack. The return value will be
used to determine whether to allow an error to be pushed or logged.
Description
The return value must be one of the PEAR_ERRORSTACK_* constants.
This functionality can be used to emulate PEAR's pushErrorHandling, and
the PEAR_ERROR_CALLBACK mode, without affecting the integrity of the error stack or logging
