how to change from mysql to mysqli

Question

i change the server from php 5.2 to php 7.3 & from centos 6 to centos 7.3 , i use also smarty when i upgrade the host the code not working i cant login to admin dashboard its give error " Invalid username/password entered. " can any please help

if ($_SESSION['ADMINID'] != "" && $_SESSION['ADMINUSERNAME'] != "" && $_SESSION['ADMINPASSWORD'] != "")
{
    $redirect = $config['adminurl']."/home.php";
    header("location: $redirect");
}
else
{

if($_POST['login']!="")
{
    $adminusername = cleanit($_POST['username']);
    $adminpassword = cleanit($_POST['password']);
    if ($adminusername == "")
    {
        $error = "Error: Username not entered.";
    }
    elseif ($adminpassword == "")
    {
        $error = "Error: Password not entered.";
    }
    else
    {
        $encodedadminpassword = md5($adminpassword);
        $query="SELECT * FROM administrators WHERE username='".mysqli_real_escape_string($adminusername)."' AND password='".mysqli_real_escape_string($encodedadminpassword)."'";
        $executequery=$conn->execute($query);
        $getid = $executequery->fields['ADMINID'];
        $getusername = $executequery->fields['username'];
        $getpassword = $executequery->fields['password'];

        if (is_numeric($getid) && $getusername != "" && $getpassword != "" && $getusername == $adminusername && $getpassword == $encodedadminpassword)
        {
            $_SESSION['ADMINID'] = $getid;
            $_SESSION['ADMINUSERNAME'] = $getusername;
            $_SESSION['ADMINPASSWORD'] = $encodedadminpassword;
            $redirect = $config['adminurl']."/home.php";
            header("location: $redirect");
        }
        else
        {
            $error = "Invalid username/password entered.";
        }
    }
}

STemplate::assign('message',$message);
STemplate::assign('error',$error);
STemplate::display('administrator/index.tpl');

}

problem in below

        $encodedadminpassword = md5($adminpassword);
        $query="SELECT * FROM administrators WHERE username='".mysqli_real_escape_string($adminusername)."' AND password='".mysqli_real_escape_string($encodedadminpassword)."'";
        $executequery=$conn->execute($query);
        $getid = $executequery->fields['ADMINID'];
        $getusername = $executequery->fields['username'];
        $getpassword = $executequery->fields['password'];