<?
ini_set("display_errors","0");
session_start();

if(isset($_POST['username'])){
$_SESSION['username'] = $_POST['username'];
}
	if(isset($_SESSION['username']))
	{
		echo 'Merhaba '.$_SESSION['username'] . '! <a href="cik.php">Çıkış</a>' ;
	}
	
	else
	{
		echo '<form action="ilk.php" method="post">Username <input type="text" name="username"><input type="submit" value="Ok"></from> ';
	}
?>

<? 
session_start();
ob_start();
$users = array("alper","tolga","alptug");
$durum = $_POST['username'];
if(isset($durum)){
	if(in_array($durum,$users)){
		echo "onaylı";
	}
	else { echo "izinsiz giriş."; }
}
	
	else
	{
		echo "izinsiz giriş.";
	}
ob_end_flush();
?>

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Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/domain/domains/domainadı.com/public_html/son.php:1) in /home/domain/domains/domainadı.com/public_html/son.php on line 2
izinsiz giriş.