php取得表單值出錯

<?php
$index = 1;
$filename = $_POST[&#39;filename&#39;];
echo $filename;
?>

<?php
$db = substr($string[0],14) . "_" . substr($string[1],14) . "_db.txt";
?>
<input type="hidden" name="filename" value="<?php echo $db; ?>">

if(isset($_POST['filename'])){
    $filename = $_POST['filename'];
}if(isset($filename)){ 
    echo $filename;
}

error_reporting = E_ALL & ~E_NOTICE

function _get($str){ 
$val = isset($_GET[$str]) ? $_GET[$str] : null; 
return $val; 
}