How should I resolve it?

<?php 
ob_start();
session_start();
$tabMessage = "PBS";
mysqli_connect("localhost","root","");
mysqli_select_db($tabMessage, "pbsorgpk_pbs");
//mysql_connect("localhost","root","");
//mysql_select_db("ws_pbs");


function checkUser($id)
{
    $Q="SELECT user_id FROM pbs_user WHERE user_id='$id'";
    $myQ=mysql_query($Q);
    $myRows=mysql_num_rows($myQ);
    
    if($myRows<1)
    {
        unset($_SESSION["identy"]);
        header('Location: index.php');
        exit();
    }
}

$securitycode='DT53)GNe{~,wd`/-';
$CAPTCHA_SECRET='6LfKHRQUAAAAANJzAy_zmS1vfIXIyX3ywu0ytqKu';
$CAPTCHA_SITEKEY='6LfKHRQUAAAAABD96PoyeYlF57eGaPKJ7uyQkKRQ';

?>