imagefill can't deal with alpha colors, use imagefilledrectangle instead.

<?php

    header("Content-Type: image/png");
    $im = imagecreatefrompng("img/button.png");
    if (empty($_GET['alpha']))
        {$_GET['alpha'] = 10;}
    $color = imagecolorallocatealpha($im, 255, 255, 255, $_GET['alpha']);
    imagefillalpha($im, $color);
    imagepng($im);
    imagedestroy($im);

    function ImageFillAlpha($image, $color)
    {
        imagefilledrectangle($image, 0, 0, imagesx($image), imagesy($image), $color);
    }

?>