error occurred in getting checkbox value

That looks like code from view file, not from controller, however because you are using name="certid[]", it's likely that you are looping over 0-6 indexes, where POST variable only sends values that were checked, so it might only have 1 value in certid array, if only one box was ticked.

Check your POST data that comes in.

controller file also attached.
output 
Array
(
   [0] => 1_2
   [1] => 2_2
   [2] => 3_2
   [3] => 4_2
   [4] => 5_2
   [5] => 6_
   [6] => 7_
)