php ติดต่อฐานข้อมูล ผมพลาดตรงไหนหรือเปล่าครับ

แบบนี้ครับ
<?php
//connect database
         $host = "localhost";
        $user = "u537968177_admin";
        $pass = "pppppp78";
        $dbname = "u537968177_regis";
        
        $conn = @mysql_connect($host,$user,$pass) or die ("Can't connecting to database");
        @mysql_select_db($db_host) or die ("Can't connecting tp table");
        //check register
        
        $username = $_POST['username'];
        $password = $_POST['password'];
        $repassword = $_POST['repassword'];
        $name = $_POST['name'];
        $sirname = $_POST['sirname'];
        $age = $_POST['age'];
        $phone = $_POST['phone'];
        $address = $_POST['address'];
        //check null & password
        
        if ($username =="" or $password =="" or $name =="" or $sirname ==""
         or $age =="" or $phone =="" or $address =="") {
          echo "please no missing";
         } else if($password != $repassword){
echo “รหัสผ่านไม่ตรงกัน”;
} else {
echo “รหัสผ่านตรงกัน”;
}
//save to database

$sql = "select * from tb_register where username='username' ";
        $result=mysql_db_query($dbname,$sql);
        $num=mysql_num_rows($result);
        if ($num>0) {
          echo "<h3>ERROR: Username as again </h3>"; exit();
        }
        $sql="insert into tb_register
        values('','$username','$password','$repassword','$name','$sirname','$age','
        $phone','$address')";
        $result=mysql_db_query($dbname,$sql);
        if ($result) {
          echo "<h3>save successfully</h3>";
          echo "<a href='register.php'></h3>
          <br><br>";
        } else {
          echo "Can't register";
        }
        mysql_close();

?>
ขึ้น server จริงครับ ของ hostinger.in.th รบกวนท่านผู้รู้ช่วยตอบด้วยนะครับ