What happened to this login module?

What happened to this login module? The database clearly has something that cannot be read. the source code is as follows: it always shows an account and password error. why ;? Php checks whether the user has logged on to session_start (); if (! Empty ($ _ SESSION [username]) {echo & quot; you have logged on to the page! & Quot; exit;} echo & what happened to this login module?
The database clearly has something that cannot be read. the source code is as follows: the account and password are always displayed incorrectly.
// Determine whether the user has logged on
Session_start ();
If (! Empty ($ _ SESSION ['username'])
{
Echo "you have logged on to the page! ";
Exit;
}
// Echo "Hello,". $ _ SESSION ['username'];
?>
Require_once ('check. php'); // introduce public files to check user input and prevent SQL injection vulnerability code
// The trim () function can intercept leading and trailing spaces
$ Username = trim ($ _ POST ['username']);
$ Pwd = md5 ($ _ POST ['pwd']);


// Set an error variable to check for errors
$ Errmsg = '';
If (empty ($ username) | empty ($ _ POST ['pwd'])
{
$ Errmsg = 'input data is incomplete ';

}
If (! Empty ($ username) & empty ($ errmsg ))
{


Include_once (conn. php );
If (! $ Conn)
{
$ Errmsg = "database connection error ";
}
// $ SQL = mysql_query ("select * from tb_yonghu ");
$ SQL = mysql_query ("select * from tb_yonghu where name = '$ username' and password =' $ pwd '");
$ Info = mysql_fetch_array ($ SQL );
$ Row = mysql_num_rows ($ SQL );
If (! $ Row ){
$ Errmsg = "incorrect account or password ";
}
Else {

// Use session to save the current user
Session_start ();
$ _ SESSION ['username'] = $ username;

// You can use the redirection function mentioned above to go to the home page
$ Errmsg = "logon successful! ";
// Update user login information
$ Ip = $ _ SERVER ['remote _ ADDR ']; // Obtain the client IP address
$ SQL = "update tb_yonghu set f_logintimes = f_logintimes + 1, f_lasttime = now (), f_loginip = '$ IP' where name =' $ username '";
Mysql_query ($ SQL );
}

// Close the database connection
Mysql_close ($ conn );
}

?>



User Login