Capital-Case Words

<?php
    if (isset($_GET['text'])) {
        $text = $_GET['text'];
        preg_match_all('/\w+/', $text, $words);
        $words = $words[0];

        $upperWords = array_filter($words, function ($word){
            return strtoupper($word) == $word;
        });
        echo implode(', ',$upperWords);
    }
?>
<form>
    <textarea rows="10" name="text"><?=$upperWords?></textarea><br>
    <input type="submit" value="Extract">
</form>