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In PHP, the operator used to create the union of two arrays is the addition operator, represented by the plus sign. When you apply this operator between two arrays, it effectively combines the elements of both arrays without duplicating the values of the first array if they are also present in the second. If there are any string keys in the arrays, the values from the second array will overwrite those from the first in the case of key collisions.

For example, if you have two arrays like this:

```php

$array1 = array("a" => "apple", "b" => "banana");

$array2 = array("b" => "blueberry", "c" => "cherry");

$result = $array1 + $array2;

```

The result will be:

```php

array("a" => "apple", "b" => "banana", "c" => "cherry");

```

In this case, the value "banana" from `$array1` is retained, while "blueberry" from `$array2` is ignored because of the existing key "b." Thus, the addition operator creates a union that preserves the values from the first array for any overlapping keys.

The other options represent comparison operators,