how to change from mysql to mysqli - php

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'];