https://www.farinspace.com/php-array-combinations/
https://www.outlandish.com/blog/xml-to-json/
https://ericlbarnes.com/2015/04/04/laravel-array-validation/
https://medium.com/@rtheunissen/efficient-data-structures-for-php-7-9dda7af674cd#.l3v3q82ya
http://shiflett.org/blog/2011/jun/sorting-multi-dimensional-arrays-in-php