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Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at C:\xampp\htdocs\student\admin\index.php:3) in C:\xampp\htdocs\student\admin\config.php on line 13

<?php
//podaci o bazi
$dbhost = "localhost";
$dbname = "baza";
$dbuser = "user";
$dbpass = "";

//konektovanje na bazu
mysql_connect ( $dbhost, $dbuser, $dbpass)or die("Could not connect: ".mysql_error());
mysql_select_db($dbname) or die(mysql_error());

//start sesija
session_start();

$user = $_POST['user'];
$pass = $_POST['pass'];

$query = "select * from sheet1 where user='$user' and pass='$pass'";
mysql_real_escape_string($user);
mysql_real_escape_string($pass);
$result = mysql_query($query);

if (mysql_num_rows($result) != 1)
{
$error = "greska prilikom logovanja";
    echo "login.php";
    
    } else {
    $_SESSION['user'] = "$user";
    include "students_list.php";
    mysql_real_escape_string($_POST['user']);
    mysql_real_escape_string($_POST['pass']);
}

?>

<?
}
else {
echo ("Pogresno korisnicko ime ili lozinka!");
}
}
else {
echo "<form name='form1' method='post' action='?login=true'>
Login name:<br><input type='text' name='user'>
<br>
Password:<br><input type='password' name='pass'>
<br>
<input type='submit' name='Login' value='Submit'>
</form>";
}
?>