Union of $a and $b :
array(3) { ["a"]=> string(5) "apple" ["b"]=> string(6) "banana" ["c"]=> string(6) "cherry" }
Union of $b and $a :
array(3) { ["a"]=> string(4) "pear" ["b"]=> string(10) "strawberry" ["c"]=> string(6) "cherry" }