ResizeImage.php

<?php
function resizeImage($filename, $max_width, $max_height)
{
    list($orig_width, $orig_height) = getimagesize($filename);

    $width = $orig_width;
    $height = $orig_height;

    # taller
    if ($height > $max_height) {
        $width = ($max_height / $height) * $width;
        $height = $max_height;
    }

    # wider
    if ($width > $max_width) {
        $height = ($max_width / $width) * $height;
        $width = $max_width;
    }

    $image_p = imagecreatetruecolor($width, $height);
    imageAlphaBlending($image_p, false);
    imageSaveAlpha($image_p, true);

    $image = imagecreatefrompng($filename);
    imageAlphaBlending($image, false);
    imageSaveAlpha($image, true);

    imagecopyresampled($image_p, $image, 0, 0, 0, 0, $width, $height, $orig_width, $orig_height);

    return $image_p;
}
?>