Input/Output Learning Mode

Consider the following script. When you run it, you obtain the output 1, 1, even though the file test.txt has been deleted by your call to unlink() as expected. Which function would you add before the last call to file_exists() to ensure that this problem will not repeat itself?

<?php
$f = fopen ("test.txt", "w");
fwrite ($f, "test");
fclose ($f);
echo (int) file_exists("test.txt") . ', ';
unlink ("c:\\test.txt");
echo (int) file_exists ("test.txt");
?>