Php Checkboxes

There quite a few checkboxes on my pages 

x.php 

If i "check" any of them, it's able to update the tables successfully and add it to the database. I use array names for the checkboxes i.e. 

<input type="checkbox" name="prob[]" value="hello"> Hello</input>

However if i dont' select anything, upon hitting Submit, I would expect the array to be null or empty. However any reference i make to prob returns a message saying 

Undefined variable: prob in /path on line 10

The checks i have tried so far are 

if (strcmp($prob[0],"") != 0)

if($prob != null) 


How does php deal with no checkbox selection ? There might be a very simple answer to it. 

Thanks,



Regards,
Jerry L
http://www.brezosoft.com