List files in folder sub-directories

I have a script that when clients create a pdf file, it is placed in a folder named with there own code. This is in a main folder called 'destcerts' and the folder structure ends up looking like the illustration below. What I need to do as admin is have access to these files and the only way I can do that at the moment is to hard code the sub-directory in the code and that is not practical for obvious reasons.

What I thought I could do is, have a select dropdown with a list of clients which when selected could trigger perhaps a change event which would say loop for the chosen client and display the files. Unfortunately, my php or jquery is not good enough to achieve this and I would be grateful if someone could show me the way to g forward with this. Many thanks

<div id="viewCerts" style="display:none;">
      <?php
        $sub = 'destcerts/';
        
        // READ THE NAMES OF FILES IN THE SUB-DIRECTORY
        $fff = new DirectoryIterator($sub);
        $sss = array();
        
        foreach ($fff as $filedata)
        {
        // SKIP THE "DOT" FILES
        if ($filedata->isDot()) continue;
        
        // ACTIVATE THIS LINE TO RESTRICT IT TO PDF FILES ONLY
        if ($filedata->getExtension() != 'pdf') continue;
        
        // SET AN INDICATOR TO SHOW WE FOUND DATA
        
        
        // CREATE LINKS TO THESE FILES
        $nom = $filedata->getFilename();
        //$value = substr ($nom, 0, 4);
        $_SESSION['value'] = $nom;
        //if($value == 'DEMO') {
        
        $lnk
        = '<img src="destcerts/PDF_icon_100.png" style="margin-bottom: 15px; margin-top:15px;"><br /><a href="'
        . $sub
        . '/'
        . $nom
        . '" style="color:#0099FF; text-decoration:none; font-size:12px; font-family: Verdana, Geneva, sans-serif;">'
        . $nom
        . '</a>'
        ;
        
        //} 
        
        // COLLECT THE LINKS HERE
        $sss[] = $lnk;
        
        }
        
        // ACCUMULATE THE TABLE ROWS HERE
        $trs   = NULL;
        
        // COLLECT GROUPS OF FOUR
        If(!empty($sss)){
        while (!empty($sss))
        {
        $td1 = array_shift($sss) or NULL;
        $td2 = array_shift($sss) or NULL;
        $td3 = array_shift($sss) or NULL;
        $td4 = array_shift($sss) or NULL;
        
        // USE HEREDOC TO INSERT THESE INTO A TABLE ROW
        $tr  = <<<EOD
        <tr>
        <td align="center" width="20%" style="padding-bottom:20px !important;">$td1</td>
        <td align="center" width="20%" style="padding-bottom:20px !important;">$td2</td>
        <td align="center" width="20%" style="padding-bottom:20px !important;">$td3</td>
        <td align="center" width="20%" style="padding-bottom:20px !important;">$td4</td>
        </tr>
EOD;
        
        // APPEND THE TABLE ROW TO THE OTHER ROWS
        $trs .= $tr;
        }
        }
        else{
        
        $msg = "There are no files to display";
      
        $tr  = <<<EOD
        <tr>
        <td style="text-align:center; padding: 10px !important; font-weight: normal;">$msg</td>
        </tr>
EOD;
        
        $trs .= $tr;
        }
        
        // USE HEREDOC TO INSERT THE TABLE ROWS INTO THE TABLE
        $tab = <<<EOD
        <table id="pdfDownload" width="97%" align="center" border="1" cellspacing="10" cellpadding="0" style="border:1px solid grey; padding-bottom: 10px; margin-bottom:20px;">
        <th style="text-align:center; padding: 10px !important; padding-bottom: 20px; border:1px solid black; background-color: #3399FF; color: white; font-size: 18px !important;" colspan="4">Destruction Certificates Download</th>
        <tr>
        <th></th>
        </tr>
        $trs
        </table>
EOD;
        
        // SHOW THE WORK PRODUCT
        echo $tab;
?>
    </div>

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.
..
destcerts
  client1
  client2
  client3
  etc

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