CREATE TABLE users (
    user_id INT(11) AUTO_INCREMENT PRIMARY KEY,
    username VARCHAR(50) NOT NULL,
    UNIQUE KEY (username)
);

CREATE TABLE contents (
    content_id INT(11) AUTO_INCREMENT PRIMARY KEY,
    content TEXT NOT NULL,
    time DATETIME NOT NULL,
    user_id INT(11) NOT NULL,
    mentioned_user VARCHAR(50),
    FOREIGN KEY (user_id) REFERENCES users(user_id),
    FOREIGN KEY (mentioned_user) REFERENCES users(username)
);

$query = "INSERT INTO users (username) VALUES ('John Doe')";

$content = "Hello @JaneDoe, how are you?";
$mentioned_user = "JaneDoe";
$user_id = 1;

$query = "INSERT INTO contents (content, time, user_id, mentioned_user) VALUES ('$content', NOW(), $user_id, '$mentioned_user')";

$content = "Hello @JaneDoe, how are you?";
$link = preg_replace("/@([\w\d]+)/", "<a href='user.php?username=$1'>@$1</a>", $content);

echo $link;