这项工作是

$msgid = $_GET['msgid'];

$oldMessage = '';

$deletedFormat = '';

//read the entire string
$str=file_get_contents('msghistory.txt');

//replace something in the file string - this is a VERY simple example
$str=str_replace($oldMessage, $deletedFormat,$str);

//write the entire string
file_put_contents('msghistory.txt', $str);