Description:
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Para 3 of the text reads:
"If only one [associative] array is given ... duplicate entries will be merged into the last one. See example three for details."

And example 3 shows:

$array_two = array("jay" => "bob", "randal" => "dante", "jay" => "jason");
$result_two = array_merge($array_two);
print_r($result_two);

Since when has an associative array been able to contain duplicated keys ("jay", in this case)? The array_merge() in the given example is a complete no-op, as a print_r($array_two) shows.

Since "duplicate entries" never occur in associative arrays, there doesn't seem to be much point discussing what happens when array_merge() encounters them.