Concerning the affirmation:
If you want to append array elements from the second array to the first array while not overwriting the elements from the first array and not re-indexing, use the + array union operator
Clearing the fact (it means ...):
If the first array have a key and a value it will not be overlap by the new array. therefore if you have an array like [1=>"alpha", 2=>"beta"] and you got a new array telling [1=>"Alpha", 3=>"Gamma"] the final array will be [1=>"alpha", 2=>"beta", 3=>"Gamma"]. The values of first array will not be modified in the result array.
So, if you are building a concatenation array where the values sometimes overlaps each other keys and you must preserve values you better use array_merge instead "plus" sign
