问使用php，如何在不覆盖文本文件开头的情况下插入文本？

EN

$file=fopen(date("Y-m-d").".txt","a+") or exit("Unable to open file!");

if ($_POST["lastname"] <> "")
{
   fwrite($file,$_POST["lastname"]."\n");
}

fclose($file);

$old_content = file_get_contents($file);
fwrite($file, $new_content."\n".$old_content);

function finsert($handle, $string, $bufferSize = 16384) {
    $insertionPoint = ftell($handle);

    // Create a temp file to stream into
    $tempPath = tempnam(sys_get_temp_dir(), "file-chainer");
    $lastPartHandle = fopen($tempPath, "w+");

    // Read in everything from the insertion point and forward
    while (!feof($handle)) {
        fwrite($lastPartHandle, fread($handle, $bufferSize), $bufferSize);
    }

    // Rewind to the insertion point
    fseek($handle, $insertionPoint);

    // Rewind the temporary stream
    rewind($lastPartHandle);

    // Write back everything starting with the string to insert
    fwrite($handle, $string);
    while (!feof($lastPartHandle)) {
        fwrite($handle, fread($lastPartHandle, $bufferSize), $bufferSize);
    }

    // Close the last part handle and delete it
    fclose($lastPartHandle);
    unlink($tempPath);

    // Re-set pointer
    fseek($handle, $insertionPoint + strlen($string));
}

$handle = fopen("file.txt", "w+");
fwrite($handle, "foobar");
rewind($handle);
finsert($handle, "baz");

// File stream is now: bazfoobar