Description:
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From manual page: https://php.net/language.operators.array
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It is only a problem in description.

In the docs, it states that $a + $b is Union and it is Union of $a and $b.
But it is not completely reflecting the characteristic of the Union in PHP.
Since strictly speaking, an Union should be mutually inclusive of different things.

In PHP, it is only an union of key regardless of value.
So it should be written as

Union of the key in $a and $b and based on $a or vice versa if it is $b + $a. (Because when there is/are key(s) exist(s) in the first variable, the same key(s) in the second variable will be ignored)


Test script:
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$a = array('something' => 'interesting', 2);
$b = array('another' => 'interesting thing', 0 => 3);
$c = $a + $b;
$d = $b + $a;

print_r($c);
print_r($d);