Why can't I clean up a file with php?

Question

linux

There is this code:

<?php
$fo = fopen("cookie.txt", "w");
fclose($fo);
?>

stuck in cron

*/1 * * * * /usr/bin/php /home/polzak/ful/index.php

rights set the rights to the file like this:
suod chmod 777 ~/ful/cookie.txt
but the file is not cleaned, why?
PS
Linux server-home 4.9.0-4-686-pae #1 SMP Debian 4.9.51-1 (2017-09-28) i686 GNU/Linux

Answer 1

And why should it be cleared, you just tear it off for writing, having received the descriptor and immediately close it.
You can write an empty string to it using the fwrite() function.

<?php
$fo = fopen("cookie.txt", "w");
fwrite($fo, "");
fclose($fo);
?>

Answer 2

Or easierfile_put_contents('cookie.txt', '');