<?php
$filename = "deneme.txt";
$birincisatir = "1 satıra ekle";
$sonuncusatir = "son satıra ekle";

$kaynak = file_get_contents($filename);

file_put_contents($filename,$birincisatir);
file_put_contents($filename,$kaynak,FILE_APPEND);
file_put_contents($filename,$sonuncusatir,FILE_APPEND);

echo file_get_contents($filename);