Description:
------------
This is part of our efforts to get Drupal 8 green on PHP 7, see https://www.drupal.org/node/2454439.

This might be related to https://bugs.php.net/bug.php?id=69371, which was a somewhat similar php bug that we found (Similar as in, arrays/array functions behaving strangely).

We have a code line like this:

array_count_values($form_state->getValue('prefix'))

This displays:
array_count_values(): Can only count STRING and INTEGER values!	Warning	NegotiationUrlForm.php	183	Drupal\language\Form\NegotiationUrlForm->validateForm()

But the array definitely is valid:
Array
(
    [en] => 
    [fr] => fr
)

(en is an empty string, type string)

Looping over the array works, accessing the keys works. Trying the same with that array works:

$ php7 -r '$array = array("en" => "", "fr" => "fr"); var_dump(array_count_values($array));'
array(2) {
  [""]=>
  int(1)
  ["fr"]=>
  int(1)
}

Re-creating the array with the same values works:

    $prefix = ['en' => $form_state->getValue('prefix')['en'], 'fr' => $form_state->getValue('prefix')['fr']];

    $count = array_count_values($prefix);

So, similar in the other issue, the array or one of those values seems to be in a weird state.

I've tracked down the place where this warning is displayed to https://github.com/php/php-src/blob/master/ext/standard/array.c#L3017. If anyone can tell me what to put there to debug the content and type of entry, then I'm more than happy to try that.