Bueno, he puesto así, pero no funciona:
Código PHP:
<?php
echo "<form action='zip.php' method='post' /><input type='checkbox' name='colores' value='rojo'>" . "<img src='./red.gif' />" . "<input type='checkbox' name='colores' value='green'>" . "<img src='./green.gif' />" . "<input type='checkbox' name='colores' value='blue'>" . "<img src='./blue.gif' /><input type='submit' value='Descargar'></form>";
?>
<?php
require ("./zipfile.php");
$zipfile = new zipfile();
$zipfile->add_dir("http://www.forosdelweb.com/images/");
foreach( $_POST['colores'] as $check ) {
$zipfile->add_file(implode("",file("" . echo $check . ".gif")), "img/01.gif");
}
header("Content-type: application/octet-stream");
header("Content-disposition: attachment; filename=fotos.zip");
echo $zipfile->file();
?>