这是使用file_put_contents在PHP中的文件权限还是编码错误？

So what I was trying to do was to build a custom CMS using PHP. I pretty much had it in mind that I would store the HTML/Javascript strings and then concatenate them together to form a new page, then using PHP with file_put_contents and directory explore, etc... this would work.

I was trying a basic example out and this wasn't overwriting the file:

There are two files, change_file.php is the one with the code below, and test-file.txt is a simple text file with the words "Test file" inside.

<?php
  error_reporting(-1);

  // simple test
  $file = 'test-file.txt';
  $mod_file = file_get_contents($file);
  $mod_file .= "appended";              // appending line
  file_put_contents($file, $mod_file);
  echo $mod_file;
?>

So I'd run the above once which would output "Test file appended", and then comment out the appending line, and refresh the page, but it stills shows "Test file" there are no errors.

Am I missing something obvious here? Like I can't just write to a public directory without permissions or something? This is in /var/www/html/test-folder