Email Activation

function register_user($register_data) {
    $actual_link = 'http://'.$_SERVER['HTTP_HOST'].$_SERVER['PHP_SELF'];
    $path_parts = pathinfo($actual_link);
    array_walk($register_data, 'array_sanitize');
    $register_data['password'] = md5($register_data['data']);

    $fields = '`' . implode('`, `', array_keys($register_data)) . '`';
    $data = '\'' . implode('\', \'', $register_data) . '\'';

    mysql_query("INSERT INTO users ($fields) VALUES ($data)");
    $activation_link = '$path_parts[\'dirname\']/activate.php?email=" . $register_data[\'email\'] . "&email_code=" . $register_data[\'email_code\'] . "';
    email($register_data['email'], 'Activate Your Account', "Hello " . $register_data['first_name'] . ",\n\nYou need to activate your account so please use the link below:\n\n $activation_link\n\n- International World Alliance Human Resource Department
    ");