Retrieve Files from remote server based on yesterday's date
===

I need to modify this script to retrieve only the files with yesterday's date. Here is what I have:

    <?php
    $username = 'XXXXXX';
    $password = 'XXXXXX';
    $ftp_server = 'data.mywebsite.com';

     $conn = ftp_connect($ftp_server) or die("Couldn't connect to $ftp_server");
    if(ftp_login($conn, $username, $password))
    {
        echo 'Logged in';
    }
    else
    {
        echo 'FTP Error:Could not log in to '.$ftp_server;
        exit();
    }
    ftp_pasv ($conn, true);
    if (ftp_chdir($conn, "../Photos/Hi-res")) {
        echo "Current FTP directory is now: " . ftp_pwd($conn) . "\n";
    } else { 
        echo "Couldn't change to Photos directory\n";
    }

    $list = ftp_nlist($conn, '.');
    function is_img($file) {
        if(preg_match('/.*\.png/', $file))
        {
            return preg_match('/.*\.png/', $file) > 0;
        }
        if(preg_match('/.*\.jpg/', $file))
        {
            return preg_match('/.*\.jpg/', $file) > 0;
        }
        if(preg_match('/.*\.gif/', $file))
        {
            return preg_match('/.*\.gif/', $file) > 0;
        }
    }

    $filtered = array_filter($list, is_img);
    foreach($filtered as $img) {
        if (ftp_get($conn, $img, $img, FTP_BINARY)) {
            echo "Successfully written to $img\n";
        } else {
            echo "There was a problem\n";
        }
    }
    ftp_close($conn);

When I run the script, it is grabbing all files not already on my server which is not what I want. I just want it to grab all the files with yesterdays date.
Thanks