functions.php

<?php

/**
 * Return the depth of an array
 * Source: https://stackoverflow.com/a/263621
 */
function array_depth(array $array) :int {
    $max_indentation = 1;

    $array_str = print_r($array, true);
    $lines = explode("\n", $array_str);

    foreach ($lines as $line) {
        $indentation = (strlen($line) - strlen(ltrim($line))) / 4;

        if ($indentation > $max_indentation) {
            $max_indentation = $indentation;
        }
    }

    return ceil(($max_indentation - 1) / 2) + 1;
}

function processRecipientChoices(array $recipientChoices) :array {
    $depth = array_depth($recipientChoices);

    $choices = [
        'addresses' => [],
        'users' => [],
        'domains' => []
    ];

    $usersEqual = true;
    if ($depth == 1) {
        $choices['addresses'] = $recipientChoices;
    } elseif ($depth == 2) {
        $choices['domains'] = array_keys($recipientChoices);
        foreach ($choices['domains'] as $domain)  {
            foreach ($choices['domains'] as $domain2)  {
                if ($recipientChoices[$domain] != $recipientChoices[$domain2]) {
                    $usersEqual = false;
                    $choices['users'] = [];
                    break 2;
                } else {
                    $choices['users'] = $recipientChoices[$domain];
                }
            }
        }

        foreach ($choices['domains'] as $domain) {
            foreach($recipientChoices[$domain] as $user) {
                if (!empty($domain)) {
                    $choices['addresses'][] = $user . '@' . $domain;
                } else {
                    $choices['addresses'][] = $user;
                }
            }
        }
    } else {
        throw new Exception("There are to many levels for custom recipient choices available. Maximum is 2", 1);
    }

    return $choices;
}