if and only if the arrays have different keys, you can use the +
operator to union the two arrays. If the arrays contain the same key (eg. default indexes), only the first one will be kept and the rest will be omitted.
eg:
$arr1 = array("color1" => "red", "color2" => "blue");
$arr2 = array("color1" => "black", "color3" => "green");
$arr3 = $arr1 + $arr2; //result is array("color1" => "red", "color2" => "blue", "color3" => "green");