Merge two arrays and retain only unique values.
Append values from second array.
Do not care about keys.

<?php
$array1 = [
    0 => 'apple',
    1 => 'orange',
    2 => 'pear',
];

$array2 = [
    0 => 'melon',
    1 => 'orange',
    2 => 'banana',
];

$result = array_keys(
    array_flip($array1) + array_flip($array2)
);
?>

Result:
[
  [0] => "apple",
  [1] => "orange",
  [2] => "pear",
  [3] => "melon",
  [4] => "banana",
}