<?php
if(isset($_POST['isim'])) {
$baglan = fopen("b.php", "w");
fwrite($baglan, $_POST['isim']);
fclose($baglan);
echo "dosyaya yazildi.";
} else { ?>
<form method="post" action="a.php">
<input type="text" name="isim">
<input type="submit" value="Ekle">
</form>
<?php } ?>