Various small fixes (#4795)

Various small fixes (#4795)

* Use strict comparison to avoid 0 == false
* Avoid unecessary loop
* Avoid undefined variable
* Fix wrong variable name
* Don't try to use the $status_info variable if it does not exist
* Avoid undefined variable
* Check that variable is defined before trying to use it
* Don't cast $data array to boolean
* Fix undefined array
* $reactivate_group variable is not always defined
* Variable $found is not always defined
* Remove unused return statement
* Fix undefined array
* api_unique_id_to_nurl() requires an integer as argument
* Don't try to use $uinfo if it is false
* Don't use the same variable names in inner loop
* Throw an exception if $fileext is not defined