List and Sort Files with PHP DirectoryIterator

Placing your valid files into an array with various columns you can sort by is probably the best bet, an associate one at that too.

I used asort() "Sort an array and maintain index association" with I think best fits your requirements.

if (is_dir($path)) 
{
    $FoundFiles = [];

    foreach (new DirectoryIterator($path) as $file) 
    {

       if ($file->isDot())
       {
          continue;
       }
    
       $fileName = $file->getFilename();

       $pieces    = explode('.', $fileName);
       $date      = explode('-', $pieces[2]);
                
       $filetypes = [ "pdf", "PDF" ];
       $filetype  = pathinfo( $file, PATHINFO_EXTENSION );

       if ( in_array( strtolower( $filetype ), $filetypes )) 
       {
          /** 
           *  Place into an Array
          **/
          $foundFiles[] = array( 
              "fileName" => $fileName,
              "date"     => $date
          );       
       }
    }
 }

Before Sorting

print_r( $foundFiles );

Array
(
   [0] => Array
       (
            [fileName] => readme.pdf
            [date] => 22/01/23
       )

   [1] => Array
       (
            [fileName] => zibra.pdf
            [date] => 22/01/53
       )

    [2] => Array
       (
            [fileName] => animate.pdf
            [date] => 22/01/53
       ) 
)
   

        

After Sorting asort()

/** 
 *   Sort the Array by FileName (The first key)
 *   We'll be using asort()
**/ 
asort( $foundFiles );

/** 
 *   After Sorting 
**/ 
print_r( $foundFiles );

Array
(
    [2] => Array
        (
            [fileName] => animate.pdf
            [date] => 22/01/53
        )

    [0] => Array
        (
            [fileName] => readme.pdf
            [date] => 22/01/23
        )

    [1] => Array
        (
            [fileName] => zibra.pdf
            [date] => 22/01/53
        )
 )

Then for printing with HTML after the function completes - Your code did it whilst the code was in a loop, which meant you couldn't sort it after it's already been printed:

<ul>
   <?php foreach( $foundFiles as $file ): ?>
      <li>File: <?php echo $file["fileName"] ?> - Date Uploaded: <?php echo $file["date"]; ?></li>
   <?php endforeach; ?>
</ul>

Use scandir instead of DirectoryIterator to get a sorted list of files:

$path = "../images/";

foreach (scandir($path) as $file){

    if($file[0] == '.'){continue;}//hidden file

    if( is_file($path.$file) ){

    }else if( is_dir($path.$file) ){

    }
}

scandir is case sensitive. For case insensitive sorting, see this answer: How do you scan the directory case insensitive?