Question

I’m comparing arrays with array_diff_key and array_diff_ukey but the output is different from both arrays. Manual says there is not difference b/w both function except the later one accpet a callback but I got the difference in output.

$array1 = array("a" => "green", "red", "blue", "red");
$array2 = array("b" => "green", "yellow", "");

print_r($result = array_diff_key($array1, $array2));

$result = array_diff_ukey($array1, $array2, function($key1, $key2) {
                                    if ($key1 == $key2)
                                        return 0;
                                    elseif ($key1 > $key2)
                                        return 1;
                                    else
                                        return -1;
                    });

print_r($result);

Output:

Array
(
    [a] => green
    [2] => red
)
Array
(
    [2] => red
)

Answer 1

It’s because you are using the == operator in your callback. When comparing strings, you should always use ===.

var_dump("b" == 0);
> true

You can see for yourself by modifying your callback to output the equal keys:

if ($key1 == $key2)
{
    echo $key1 . " == " . $key2 . " = " . var_export($key1 == $key2, true) . "<br>";
    return 0;
}

Will result in:

0 == a = true
a == 0 = true
0 == b = true
b == 0 = true
a == 0 = true
0 == b = true
1 == 1 = true

The reason being that when you compare and int vs a string with == they will both be casted to an int value; so “a” becomes 0.