rename the written text files in numbers, eg. 1,2,3...

what i want to do is when the visitor fills out the form and press submit, i want to store the information as a text file, and call it "text1.txt", when the second visitor comes and submit another form, I want the second form to call "text2.txt", and so on.... continues to name it in accending numbers, below is my code:

PHP:

<?php
$dataf = "1.txt";
$file = fopen("$dataf","w");
$write = fwrite($file,"not important text");
fclose($file);
?>

What code do I insert in if i want to rename the $dataf text file in numbers, eg. 1,2,3... in sequence, each time creating a new file with a interger above the last one written?