Resize image using PHP if it's too small

I'm allowing user uploads, and I want to scale their images开发者_运维知识库 up if they're too small (low quality is a non-issue). I need to make the smallest side become 150px and have the other dimension scale up to keep the aspect ratio. I need to make it work for .jpg, .gif and .png files.

Any pointers would be greatly appreciated, I'm struggling to find anything about making images larger like this.

---

As answered here, give it a try to WideImage.

---

Thanks to Alexander for the WideImage suggestion.

I simply used this:

require_once('WideImage/WideImage.php');
$image = WideImage::load($_FILES['image']['tmp_name']);
$resized = $image->resize(150,150,'outside','up');
$resized->saveToFile($target_file);

It worked perfectly, and by using the "up" option, it only scales images smaller than the dimensions set up, and leaves everything else.