https://www.virendrachandak.com/techtalk/php-isset-vs-empty-vs-is_null/
http://www.zomeoff.com/php-fast-way-to-determine-a-key-elements-existance-in-an-array/
- This will check to see if there is a value for an item in an array.
- This will return
trueif the value is set to''. - This will return
falseif the value is set tonull.
$people = array(