Test forms:
<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Test form</title>
</head>
<body>
<form action="index.php" method="post">
<p>Name: <input type="text" name="name"></p>
<p>E-mail: <input type="email" name="email"></p>
<p>Text: <textarea name="message"></textarea></p>
<p>Agree? <input type="checkbox" name="agree"></p>
<p>
<select name="lang[]" multiple>
<option value="ru">Russian</option>
<option value="en">English</option>
<option value="fr">France</option>
</select>
</p>
<p><button type="submit" name="send_form">Send</button></p>
</form>
<hr>
<form action="index.php" method="get">
<p>Search: <input type="text" name="s"></p>
<p><button type="submit" name="search_form" value="search">Send</button></p>
</form>
<a href="?id=1">Product 1</a>
<a href="?id=5">Product 5</a>
</body>
</html>
On index.php
foreach ($_POST['lang'] as $lang) {
var_dump($lang);
}