The relationship between PHP execution session and front-end JS

<?phperror_reporting (0); $path = './tmp/'; $sess _name = Session_name (); Echo $sess _name; $sess _id = $_get[$sess _name]; echo $sess _id;session_id ($sess _id); Session_save_path ($path); session_start (); if ((Trim ($_post[' username '))! = ' TM ' and (Trim ($_post[' password ')) = ' 111 ') {echo "<script>alert (' username and password error! '); location.href= ' index.php ' </script> ";} $_session[' admin '] = ' mrsoft ';? >

Even if the alert statement above is executed. However, the subsequent session property assignment statement will still be executed before the location jump.

The relationship between PHP execution session and front-end JS