php去掉文本的前2个字段

<?php
$filename = "dbbak.txt";
$contents = file_get_contents($filename);

$pieces = explode("\n", $contents);
$result_content="";
foreach($pieces as $v){
    $result_content.=substr($v,2);
}
file_put_contents("dbbak.php",$result_content);
echo "success";