Why is the value passed by session null?

Why is the value transmitted by session null? Member_login_do.php & lt; html & gt; & lt ;? Php & nbsp;/* & nbsp; Member Login processing page & nbsp; */& nbsp; require_once (".. why does/conn/Co use session to pass the value blank?
Member_login_do.php


/* Member Login processing page */
Require_once ("../conn/Conn_DB.php"); // contains the database link file
If ($ _ POST ["txt_username"]! = "" & $ _ POST ["txt_pwd"]! = "")
{
$ Name = $ _ POST ["txt_username"]; // Obtain the submitted user name
$ Pwd = $ _ POST ["txt_pwd"]; // Obtain the submitted password
$ Str = "select * from Member_Info where M_Name = '$ name' and M_Password =' $ pwd'"; // query statement
$ Result = mysql_query ($ str); // execute an SQL statement
If (mysql_num_rows ($ result)> 0) // view the number of returned query results
{
Session_id (SID );
Session_start (); // login successful, set the SESSION value
$ _ SESSION ['member'] = $ _ POST ['txt _ username'];
// Session_write_close ();
Echo "script window. location. href = '../member_info.php'; script";
// Exit (0 );
}
Else
{
Echo "script alert ('user name or password is incorrect! '); Window. location. href =' ../member_login.php 'script ";
}
}
Else
{
Echo "script" alert ('Enter your username and password! '); Window. location. href =' ../member_login.php 'script ";
}
?>
 Session MySQL php shared:
------ Solution --------------------
Session_member_check.php

Do you not need start?
------ Solution --------------------
Each program requires session_start (); you cannot do it only in member_login_do.php.

SID has a value only after session_start ().

------ Solution --------------------
Injection
------ Solution --------------------
If (mysql_num_rows ($ result)> 0) // view the number of returned query results
{
Exit; // is it here?
..................

First, you need to know whether to jump to the member_login.php file in member_info.php or before.