Sort files with filemtime

I have PHP file that generates gallery from a directory of images.

I want to sort images by modification date.

I'm trying to use filemtime function then sort function using the following code:

<?php
  $src_folder = 'gallery';  
  $src_files = scandir($src_folder); 

  function filetime_callback($a, $b) {
    if (filemtime($src_folder.'/'.$a) === filemtime($src_folder.'/'.$b)) return 0;
    return filemtime($src_folder.'/'.$a) > filemtime($src_folder.'/'.$b) ? -1 : 1; 
  }

  $files = array();
  usort($files, "filetime_callback");
              
  foreach($src_files as $file) {
   echo $file . ' - ' . date ("F d Y H:i:s.", filemtime($src_folder.'/'.$file)) . '<br>';
  }
?>

And the output is like this:

image01.jpg - October 22 2017 19:40:02.
image02.jpg - October 22 2017 19:39:19.
image03.jpg - October 22 2017 19:39:23.
image04.jpg - October 22 2017 19:39:28.

It is not sorted by modification date.

How can I make my code work?

function scan_dir($dir) {
      $ignored = array('.', '..', '.svn', '.htaccess');
      $files = array();
      foreach (scandir($dir) as $file) {
          if (in_array($file, $ignored))  {
              continue;
          }
          $filemtime = filemtime($dir . '/' . $file);
          $files[$file] = $filemtime;
      }

      arsort($files);
      $files = array_keys($files);

      return ($files) ? $files : false;
  }

  $src_folder = 'gallery';

  $files = scan_dir($src_folder);
  
  foreach($files as $file) {
      echo $file . ' - ' . date ("F d Y H:i:s.", filemtime($src_folder.'/'.$file)) . '<br>';
  }