[แก้ได้แล้วค่ะ ][ขอคำแนะนำ $_SESSION ไม่ทำงาน] $_SESSION['username'] มันเช็คแค่ครั้งเดียวตอน login หน้าอื่นไม่เช็ค มีวิธีแก้ยังไงค่ะ

<?php 
	ob_start();	
	@session_start();
	include'config/config.inc.php';
	
	if(( $_POST['username']=="") || ($_POST['password']=="")){

	} else {
		$query = "SELECT * FROM member WHERE user='".strtolower($_POST['username'])."' AND  password ='".$_POST['password']."'";
		$result = mysql_query($query) or die (mysql_error());
		
		if(mysql_num_rows($result) > 0 ){			
			$_SESSION['username'] = $_POST['username'];
			
		}else{
			$printchk = "<font color='#900' size='2'>ผู้ใช้งานหรือ รหัสผ่านไม่ถูกต้อง&nbsp;&nbsp;&nbsp;</font>";				
		}
	}
?>
 <?php
	if($_SESSION['username']==""){
		
?>
       <form action="" method="post">
<table style="width:65%; height:15px; padding:8px 5px ">
    <tr >
      <td style="width:20%; height:15px;  padding:8px 5px;"><input type="text" name="username" class="form-control input-sm" placeholder="username" style="height:25px; width:146px; margin-left:25px; height:25px; padding:0 0 0 5px;"></td>
      <td style="width:20%; height:15px;  padding:8px 5px;"><input type="password" name="password" class="form-control input-sm" placeholder="password"  style="height:25px; width:146px; margin-left:5px; height:25px; padding:0 0 0 5px;"></td>
 
    	
    
  <td style="width:10px; height:15px;  padding:8px 5px""><input name="" type="submit" value="เข้าสู่ระบบ"></td>
  <td colspan="2" align="right" width="10px" style=" height:15px; width:20px;  padding:8px 5px""><?=$printchk?></td>
    </tr>
</table>

     </form>
<?php
}else { 
	$query = "select * from member where user='".$_SESSION['username']."'";
	$result = mysql_query($query);
	$rs = mysql_fetch_array($result);
?>
<tr height="28">
    	<td colspan="2" align="center" ><samp style='color: rgb(255,255,255); margin-left:15px; '>ยินดีต้อนรับ</samp></td>
    </tr>
    <tr height="1">
    	<td>&nbsp;</td>
    </tr>
    <tr height="20">
    	<td colspan="2" style="font-weight:bold;">&nbsp;&nbsp;&nbsp;<samp style='color: rgb(255,255,255); '>คุณ <?=$rs[name]?></samp></td>
    </tr>&nbsp;
    <tr height="30" align="right">
<td colspan="2" style="font-size:13px; "><a href="./profile.php"><samp style='color: rgb(255,204,51); '>แก้ไขข้อมูลส่วนตัว</samp></a> <samp style='color: rgb(255,255,255); '>| </samp>
<a href="./logout.php"><samp style='color: rgb(255,204,51); '>ออกจากระบบ</samp></a>&nbsp;&nbsp;&nbsp;</td>
    </tr>
    <?Php } ?>