Why does in_array() not work on $_POST?

Question

I'm trying to check that user's submitted data, from $_POST, has at least the same elements that my passed array has. I'm doing it because I will use those elements later by calling $_POST['element'] and I don't like errors about that element doesn't exist (isn't set). :)

I don't want to use something like isset($_POST['x'], $_POST['y'], $_POST['z']) because each time I need to rewrite $_POST and it seems unreadable as well.

I tried to use in_array(array('x', 'y', 'z'), $_POST), but it doesn't work (it returns false when it should return true). Any ideas how to make that work? :) I'm sure that I have empty strings as $_POST['x'], $_POST['y'] and $_POST['z']. I even tried to change values of hose three $_POST elements to something other than empty string - still... doesn'y work as expected. :(

Thanks in an advice! :)

Edit:

Just found out that in_array() checks values, not keys. Then, I tried to do like this...

in_array(array('title', 'slug', 'content'), array_keys($_POST))

Still, it returns false. How does it comes so? ;/

Edit #2:

Okay, here are results of debugging...

Incoming $_POST:

array(3) {
    ["title"]=>
    string(0) ""
    ["slug"]=>
    string(0) ""
    ["content"]=>
    string(0) ""
}

Result of array_keys($_POST):

array(3) {
    [0]=>
    string(5) "title"
    [1]=>
    string(4) "slug"
    [2]=>
    string(7) "content"
}

Result of in_array(array('title', 'slug', 'content'), array_keys($_POST)):

bool(false)

The question... why is it false? I did all correct, as much as I know.

Edit #3:

At the end, I created my own method called Arr::keys_exists($keys, $array).

Answer 1

in_array() checks to see if a value exists in an array, not a key. If you want to check to see if a key exists, then you'd want something like...

in_array('x', array_keys($_POST));

or the simpler...

array_key_exists('x', $_POST);

---

If you want to check for many keys at once:

$required_keys = array('x'=>1, 'y'=>1, 'z'=>1);
$missing_keys = array_diff_key($required_keys, $_POST);
$missing_keys_count = count($missing_keys);

Answer 2

Because in_array checks if the needle is in the array exactly. See example #3 of the manual-page. array_key_exists cannot work with a key as first argument because array's aren't valid with arrays as keys.

You want something like all_in_array(array $needles, array $haystack); or array_all_keys_exists(array $keys, array $search); which returns whether all elements are in the array. You can probably implement something like this yourself, or ask for more help here.