PHP对png图像进行缩放,支持透明背景

function smart_resize_image( $file, $width = 0, $height = 0, $proportional = false, $output = 'file', $delete_original = true, $use_linux_commands = false )
    {
        if ( $height <= 0 && $width <= 0 ) {
            return false;
        }
        $info = getimagesize($file);
        $image = '';
 
        $final_width = 0;
        $final_height = 0;
        list($width_old, $height_old) = $info;
 
        if ($proportional) {
            if ($width == 0) $factor = $height/$height_old;
            elseif ($height == 0) $factor = $width/$width_old;
            else $factor = min ( $width / $width_old, $height / $height_old);  
            $final_width = round ($width_old * $factor);
            $final_height = round ($height_old * $factor);
 
        }
        else {       
            $final_width = ( $width <= 0 ) ? $width_old : $width;
            $final_height = ( $height <= 0 ) ? $height_old : $height;
        }
 
        switch ($info[2] ) {
            case IMAGETYPE_GIF:
                $image = imagecreatefromgif($file);
            break;
            case IMAGETYPE_JPEG:
                $image = imagecreatefromjpeg($file);
            break;
            case IMAGETYPE_PNG:
                $image = imagecreatefrompng($file);
            break;
            default:
                return false;
        }
 
        $image_resized = imagecreatetruecolor( $final_width, $final_height );
 
        if ( ($info[2] == IMAGETYPE_GIF) || ($info[2] == IMAGETYPE_PNG) ) {
            $trnprt_indx = imagecolortransparent($image);
            // If we have a specific transparent color
            if ($trnprt_indx >= 0) {
                // Get the original image's transparent color's RGB values
                $trnprt_color    = imagecolorsforindex($image, $trnprt_indx);
                // Allocate the same color in the new image resource
                $trnprt_indx    = imagecolorallocate($image_resized, $trnprt_color['red'], $trnprt_color['green'], $trnprt_color['blue']);
                // Completely fill the background of the new image with allocated color.
                imagefill($image_resized, 0, 0, $trnprt_indx);
                // Set the background color for new image to transparent
                imagecolortransparent($image_resized, $trnprt_indx);
            }
            // Always make a transparent background color for PNGs that don't have one allocated already
            elseif ($info[2] == IMAGETYPE_PNG) {
                // Turn off transparency blending (temporarily)
                imagealphablending($image_resized, false);
                // Create a new transparent color for image
                $color = imagecolorallocatealpha($image_resized, 0, 0, 0, 127);
 
                // Completely fill the background of the new image with allocated color.
                imagefill($image_resized, 0, 0, $color);
 
                // Restore transparency blending
                imagesavealpha($image_resized, true);
            }
        }
 
        imagecopyresampled($image_resized, $image, 0, 0, 0, 0, $final_width, $final_height, $width_old, $height_old);
 
        if ( $delete_original ) {
            if ( $use_linux_commands )
                exec('rm '.$file);
            else
                @unlink($file);
        }
 
        switch ( strtolower($output) ) {
            case 'browser':
                $mime = image_type_to_mime_type($info[2]);
                header("Content-type: $mime");
                $output = NULL;
            break;
            case 'file':
                $output = $file;
            break;
            case 'return':
                return $image_resized;
            break;
            default:
            break;
        }
 
        switch ($info[2] ) {
            case IMAGETYPE_GIF:
                imagegif($image_resized, $output);
            break;
            case IMAGETYPE_JPEG:
                imagejpeg($image_resized, $output);
            break;
            case IMAGETYPE_PNG:
                imagepng($image_resized, $output);
            break;
            default:
                return false;
        }
 
        return true;
    }