nine / solution 9.1

<?php

function mineEmails(string $t)
{
    $t = trim($t);
    if (!strlen($t)) {
        print_r("<p>no emails found because text was blank.</p>");
        return;
    }
    $t = mb_convert_case($t, MB_CASE_LOWER);
    preg_replace_callback("/[a-z0-9!#%\$&\'\*\+\-\/\=\?\^_`\{|\}~\.]+\@[a-z0-9\-]+(?:\.[a-z0-9\-]+)+/", "addEmail", $t);
}

function addEmail($e)
{
    global $emails;
    $e = $e[0];
    (!isset($emails) || !count($emails)) && ($emails = []);
    $e = verifyEmail($e);
    $e && strlen($e) && !in_array($e, $emails) && ($emails[] = $e) && natsort($emails);
}

function showEmails()
{
    global $emails;
    if (!isset($emails) || !count($emails)) {
        return;
    }
    print_r("<ul>");
    array_map(fn($e) => print_r("<li>" . $e . "</li>"), $emails);
    print_r("</ul>");
}

function verifyEmail(string $e)
{
    // check for basic pattern ([email protected])
    if (!preg_match("/.+@.+\..+/", $e)) {
        return false;
    }

    // convert to lowercase
    $e = mb_convert_case($e, MB_CASE_LOWER);

    // eliminate whitespace
    $e = preg_replace("/\s+/", "", $e);

    // simplify consecutive periods
    $e = preg_replace("/\.+/", ".", $e);

    // split email into name and domain because their constraints are different
    $name = explode("@", $e)[0];
    $domain = explode("@", $e)[1];

    // check for both elements
    if (!strlen($name) || !strlen($domain)) {
        return false;
    }

    // remove leading/trailing periods
    $name = trim($name, ".");
    $domain = trim($domain, ".");

    // remove invalid characters from both sections
    $name = preg_replace("/[^a-z0-9!#%\$&\'\*\+\-\/\=\?\^_`\{|\}~\.]/", "", $name);
    $domain = preg_replace("/[^a-z0-9\-\.]/", "", $domain);

    // crop name to 64 characters
    $name = substr($name, 0, 64);

    // crop each domain segment to 63 characters
    $domain = explode(".", $domain);
    foreach ($domain as &$i) {
        $i = substr($i, 0, 63);
    }
    $domain = implode(".", $domain);

    // check for both elements again (in case all characters were invalid)
    if (!strlen($name) || !strlen($domain)) {
        return false;
    }

    // rebuild email
    $e = $name . "@" . $domain;

    return $e;
}

isset($_POST) && isset($_POST["source"]) ? mineEmails($_POST["source"]) : print_r("<p>please submit text to parse.</p>");
isset($emails) && count($emails) && showEmails();