Fix "only variables should be passed by reference" PHP notice

[rvelhote]

The array_walk function accepts the array parameter as reference. That
parameter is being passed directly from the method return however only
variables can be passed as reference.

[andercampanha]

Thanks for the contribution, but passing a variable to array_walk can be dangerous since it is passed by reference and it may be modified in a unpredictable way. A better way to solve this problem would be to replace array_walk with array_map.