<?php
ini_set('display_errors',0);
$id_del_gordo = 45;
if (isset($_FILES['img'])){
    $nombre = $_FILES['img']['name'];
    $Anombre = explode('.',$nombre);
    $extension = end($Anombre);
    if (preg_match("/(?:png|jpeg|jpg|gif)$/",$extension)){
        $fecha = date("Y_m_d_H_i_s");
        move_uploaded_file($_FILES['img']['tmp_name'], "Comprobantes/{$id_del_gordo}_{$fecha}.{$extension}");
    } else {
        echo "Extensión no permitida!";
    }
}
?>
<form method="POST" enctype="multipart/form-data">
<input type='file' name='img'/><br>
<input type='submit' value='Subir'/>
</form>

<br><br><br>Fotos del gordo:<br>
<?php
if ($gestor = opendir('Comprobantes')) {
    while (false !== ($entrada = readdir($gestor))) {
        if (preg_match("/^{$id_del_gordo}_/",$entrada)){
            echo "<img src='Comprobante/{$entrada}'/><br><br>";
        }
    }
}
?>