Unable to retrieve data from php database, what could be the problem?

Question

PHP

Hello. Since once I say I'm a bad programmer.
I had such a problem when I want to display data from the database in this way, then everything works:

$res=$this->db->mysqli->query('SELECT * FROM user) or die (mysql_error());
    $data = array();
    while(($row=$res->fetch_assoc())!=false){
      $data[]=$row;
    }
    echo $data; //Получаю массив

but if I need the data of a certain user

$email='[email protected]';
    $res=$this->db->mysqli->query('SELECT * FROM user WHERE email='.$email);
    $data = array();
    while(($row=$res->fetch_assoc())!=false){//ОШИБКА
      $data[]=$row;
    }
    echo $data;

then the error
Fatal error: Call to a member function fetch_assoc() on a non-object in file path is thrown.
How can this be corrected? Thanks!)

Answer 1

email string, and strings are quoted

$res=$this->db->mysqli->query("SELECT * FROM user WHERE email='".mysql_real_escape_string($email)."'");

Answer 2

You have SQL injection. Use PDO and prepared statements. Better yet, take yii2