PHP: Make file name original by adding an underscore and a number

Using PHP, how can I automatically make a file name original by adding an underscore and a number to the end?

If you run the script 12 times, the files saved should be named:
hello.txt
hello_1.txt
hello_2.txt
hello_3.txt
hello_4.txt
hello_5.txt
hello_6.txt
hello_7.txt
hello_8.txt
hello_9.txt
hello_10.txt
hello_11.txt



<?php

file_put_contents('hello.txt','hello');

function original_name ($str){
 while(file_exists($str)) {
  // Add underscore and number after name before extention
 }
 return $str;
}

?>