Notice: Only variables should be passed by reference in

Question

Everyone,

I am facing in user.php code. It's showing this error:

Notice: Only variables should be passed by reference in line 19

Please check the below code and let me know what I am missing. I will be very thankful,

<?php
class user {
    protected $pdo;

    function __construct($pdo){
        $this->pdo = $pdo;
    }

    public function checkInput($var){
        $var = htmlspecialchars($var);
        $var = trim($var);
        $var = stripcslashes($var);
        return $var;
    }

    public function login($email, $password){
        $stmt = $this->pdo->prepare("SELECT `user_id` FROM `users` WHERE `email` = :email AND `password` = :password");
        $stmt->bindParam(":email", $email, PDO::PARAM_STR);
        $stmt->bindParam(":password", md5($password), PDO::PARAM_STR);
        $stmt->execute();

        $user = $stmt->fetch(PDO::FETCH_OBJ);
        $count = $stmt->rowCount();

        if($count > 0){
            $_SESSION['user_id'] = $user->user_id;
            header('Location: home.php');
        }else{
            return false;
        }
    }
}

Thank you!