PHP fetch_array（）函数

I can't get the result for the fetch_array function. Any idea what is happening?

    $db = new mysqli(DB_HOSTNAME, DB_USERNAME, DB_PASSWORD, DB_DATABASE);
$aa = htmlspecialchars($_POST['aa'], ENT_QUOTES);
            $sqlc = "SELECT * FROM oc_user WHERE username='".$aa."'";
            $resultc = $db->query($sqlc);

            if ($resultc === TRUE) {
            // manager email
            $mngemail = "";

                       while ($row = $resultc->fetch_array(MYSQLI_ASSOC)) {    
                            $mngemail .= $row['email'];
                       }
        echo 'email:'.$mngemail;

You need to assign the returned value from fetch_array to $row

ie:

while ($row = $resultc->fetch_array(MYSQLI_ASSOC)) {       
    $mngemail .= $row['email'];
}