<?php
if ($_POST['kaydet']) {
    file_put_contents('dosya.txt', $_POST['dosya']);
    header('Location: ' . $_SERVER['SCRIPT_NAME']);
}
?>

<form action="" method="post">
    <textarea name="dosya" style="width:600px; height:300px;"><?php echo file_get_contents('dosya.txt');?></textarea>
    <br />
    <input type="submit" style="margin-top:5px;" name="kaydet" value="Kaydet" />
</form>