Resize/crop/pad a picture to a fixed size

This solution is basically the same as Can Berk Güder's, but after having spent some time writing and commenting, I felt like posting.

This function creates a thumbnail that is exactly as big as the size you give it. The image is resized to best fit the size of the thumbnail. If it does not fit exactly in both directions, it's centered in the thumnail. Extensive comments explain the goings-on.

function thumbnail_box($img, $box_w, $box_h) {
    //create the image, of the required size
    $new = imagecreatetruecolor($box_w, $box_h);
    if($new === false) {
        //creation failed -- probably not enough memory
        return null;
    }


    //Fill the image with a light grey color
    //(this will be visible in the padding around the image,
    //if the aspect ratios of the image and the thumbnail do not match)
    //Replace this with any color you want, or comment it out for black.
    //I used grey for testing =)
    $fill = imagecolorallocate($new, 200, 200, 205);
    imagefill($new, 0, 0, $fill);

    //compute resize ratio
    $hratio = $box_h / imagesy($img);
    $wratio = $box_w / imagesx($img);
    $ratio = min($hratio, $wratio);

    //if the source is smaller than the thumbnail size, 
    //don't resize -- add a margin instead
    //(that is, dont magnify images)
    if($ratio > 1.0)
        $ratio = 1.0;

    //compute sizes
    $sy = floor(imagesy($img) * $ratio);
    $sx = floor(imagesx($img) * $ratio);

    //compute margins
    //Using these margins centers the image in the thumbnail.
    //If you always want the image to the top left, 
    //set both of these to 0
    $m_y = floor(($box_h - $sy) / 2);
    $m_x = floor(($box_w - $sx) / 2);

    //Copy the image data, and resample
    //
    //If you want a fast and ugly thumbnail,
    //replace imagecopyresampled with imagecopyresized
    if(!imagecopyresampled($new, $img,
        $m_x, $m_y, //dest x, y (margins)
        0, 0, //src x, y (0,0 means top left)
        $sx, $sy,//dest w, h (resample to this size (computed above)
        imagesx($img), imagesy($img)) //src w, h (the full size of the original)
    ) {
        //copy failed
        imagedestroy($new);
        return null;
    }
    //copy successful
    return $new;
}

Example usage:

$i = imagecreatefromjpeg("img.jpg");
$thumb = thumbnail_box($i, 210, 150);
imagedestroy($i);

if(is_null($thumb)) {
    /* image creation or copying failed */
    header('HTTP/1.1 500 Internal Server Error');
    exit();
}
header('Content-Type: image/jpeg');
imagejpeg($thumb);

This doesn't crop the picture, but leaves space around the new image if necessary, which I think is a better approach (than cropping) when creating thumbnails.

$w = 210;
$h = 150;

$orig_w = imagesx($original);
$orig_h = imagesy($original);

$w_ratio = $orig_w / $w;
$h_ratio = $orig_h / $h;

$ratio = $w_ratio > $h_ratio ? $w_ratio : $h_ratio;

$dst_w = $orig_w / $ratio;
$dst_h = $orig_h / $ratio;
$dst_x = ($w - $dst_w) / 2;
$dst_y = ($h - $dst_h) / 2;

$thumbnail = imagecreatetruecolor($w, $h);

imagecopyresampled($thumbnail, $original, $dst_x, $dst_y,
                   0, 0, $dst_w, $dst_h, $orig_w, $orig_h);

Do you have Imagick? If so you can load the image with it and do something like thumbnailimage()

There you can skip either of the parameters (height or width) and it will resize correctly.

Maybe take look at PHPThumb (it works with GD and ImageMagick)

Just a tip for high-quality fast thumbnail generation from large images: (from the php.net site)

If you do the thumbnail generation in two stages:

1. From original to an intermediate image with twice the final dimensions, using a fast resize
2. From the intermediate image to the final thumbnail using a high-quality resample

then this can be much faster; the resize in step 1 is relatively poor quality for its size but has enough extra resolution that in step 2 the quality is decent, and the intermediate image is small enough that the high-quality resample (which works nicely on a 2:1 resize) proceeds very fast.