PHP marge arrays with same key – Woocommerce

Loop through the first level of entries, then added a nest loop to create variables from the desired keys and values from the subarrays.

Because your deepest subarray only has one element in it, you can use "array destructuring" inside of the nested loop to assign the lone element value as $value.

Ensure unique values in the output by declaring the $value as the deep value’s key in the result array.

If you truly want the subarrays to be re-indexed, you can use array_map() and array_values() for this — but only do this if necessary.

Code: (Demo)

$result = [];
foreach ($array as $entries) {
    foreach ($entries as $key => [$value]) {
        $result[$key][$value] = $value;
    }
}

var_export($result);

Output:

array (
  'pa_color' => 
  array (
    'red' => 'red',
    'gray' => 'gray',
    'yellow' => 'yellow',
  ),
  'pa_modello' => 
  array (
    'modello2' => 'modello2',
  ),
)

p.s. If your deep subarrays might have more than one element, then just accommodate that with another loop to iterate that level.

$result = [];
foreach ($array as $entries) {
    foreach ($entries as $key => $values) {
        foreach ($values as $value) {
            $result[$key][$value] = $value;
        }
    }
}

Translated to the code that you wrote in your self-answering post, I think it might look like this:

global $wp_query;
$prod_attrs = [];
foreach ($wp_query->posts as $_post) {
    foreach (wc_get_product($_post->ID)->get_attributes() as $wc_attr => $wc_term_objs) {
        foreach ($wc_term_objs->get_terms() as $wc_term) {
             $prod_attrs[$wc_attr][$wc_term->slug] = $wc_term->slug;
         }
    }
}
var_export($prod_attr);