<?php
if(isset($_POST['sayi'])){
$sayi = intval($_POST['sayi']);
$sayi = 170 + $sayi;
echo "$sayi"; 
}
else{
echo '
<form action="dosya.php" method="post">
<input type="text" name="sayi"> <br/>
<input type="hidden" name="submit" value="1">
<input type="submit" value="hesapla">
</form>';
}