将java脚本值设置为php会话并将会话值转换为另一个php文件

I'm trying to set the JavaScript value to a PHP Session variable and get this session value to another php page. here my code. on the same it will show the value using alert. here's my first page index_1.php

 <Script>
  function(no_user){
   `var num_user = no_user;
    '<?php $_SESSION["num_user"] = "' + no_user+ '"; ?>';
     //alert('<?php echo $_SESSION["num_user"] ?>');*/
    window.open("demo.php"); 
  }
 </script>

another page index_2.php

<?php
if(isset($_SESSION['num_user'])){
$a = $_SESSION['num_user'];
echo "Number of user: ". $a;

?>

You can not set php variable in js try to set it another location(demo.php)

 <script>
  function(no_user){
   var num_user = no_user;
   window.open("demo.php?no_user="+no_user); 
   //or window.location = 'index2.php?no_user'+no_user;
  }
 </script>

and set session on demo.php/index2.php

session_start();
if(!empty($_GET['no_user'])) {
  $_SESSION['num_user']= $_GET['no_user'];
}

Just make sure to set session_start() in all PHP.Files before accessing $_SESSION-Variables and you're fine. PHP will automagically take care of re-using that session in other .php-files (same server, same domain etc.).