Merging two arrays with the "+" (array union operator) How does it work?

I have some code that appears to merge the data from two arrays using `+=`, but it doesn't include all of the elements in the element. How does it work? Example: $test = array('hi'); $test += array('test', 'oh'); var_dump($test); Output: array(2) { [0]=> string(2) "hi" [1]=> string(2) "oh" } What does `+` mean when used on arrays in PHP?