Is there any other way of doing this apart from using global variables.

Don't know. What are you doing? So far, you've only asked how to set a global variable. (You said class var, but that's the same thing.)

Also, I cannot seem to be able to set parent class variables in child class even with declaration using 'our'

Reread what I said: "which means you can access the array as @CommonObject::common_array from anywhere."