bool ftruncate ( resource handle , intsize )

<?php
   //check filesize
   echo filesize("/PhpProject/sample.txt");
   echo "\n";
   file = fopen("/PhpProject/sample.txt", "a+");

   ftruncate(file, 100);
   fclose($file);

   //Clear cache and check filesize again
   clearstatcache();

   echo filesize("/PhpProject/sample.txt");
?>

49
100