bootstrap changes

                   
|Q            |A  |
|---          |---|
|Bug Fix?     |n  |
|New Feature? |n  |
|BC Breaks?   |n  |
|Deprecations?|n  |
|Tests Pass?  |n  |
|Fixed Tickets|   |
|License      |MIT|
|Doc PR       |   |
                   


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How about to checking if ```$callback``` is callable?
[is_callable](http://it2.php.net/manual/it/function.is-callable.php)