Filter by Hashtag Search Button

Question

i am trying to allow users to filter comments by the hashtags used in them but i am very new to php so do not understand how to link them together (the search button in my html and search in my php)

php:

try {
    if (isset($_POST['search-submit']) && $_POST['search-submit'] == 'Search') {
        $selectedHashtag = isset($_POST['hashtag-filter']) ? $_POST['hashtag-filter'] : '';

        $stmt = $conn->prepare("SELECT commDate, ID, username, comment, hashtags FROM comments WHERE hashtags = :selectedHashtag");
        $stmt->bindParam(':selectedHashtag', $selectedHashtag);
    } else {
        $stmt = $conn->prepare("SELECT commDate, ID, username, comment, hashtags FROM comments");
    }

    $stmt->execute();

    $comments = $stmt->fetchAll(PDO::FETCH_ASSOC);

    echo '<div class="comment-container">';
    foreach ($comments as $comment) {
        echo '<div class="comment">';
        echo '<p><strong>Date:</strong> ' . $comment['commDate'] . '</p>';
        echo '<p><strong>Username:</strong> ' . $comment['username'] . '</p>';
        echo '<p><strong>Comment:</strong> ' . $comment['comment'] . '</p>';
        echo '<p><strong>Hashtag:</strong> ' . $comment['hashtags'] . '</p>';
        echo '</div>';
    }
    echo '</div>';

html:

if(isset($_POST['submit'])) {

$retrived_result = $hashtag->search_hashtag($_POST);

}
$stmt = $conn->prepare("SELECT commDate, ID, username, comment, hashtags FROM comments");
        $stmt->execute();
        $comments = $stmt->fetchAll(PDO::FETCH_ASSOC);
        
        echo '<div class="comment-container">';
        foreach ($comments as $comment) {
            echo '<div class="comment">';
            echo '<p><strong>Date:</strong> ' . $comment['commDate'] . '</p>';
            echo '<p><strong>Username:</strong> ' . $comment['username'] . '</p>';
            echo '<p><strong>Comment:</strong> ' . $comment['comment'] . '</p>';
            echo '<p><strong>Hashtag:</strong> ' . $comment['hashtags'] . '</p>';
            echo '</div>';
        }
        echo '</div>';