class.upload.php is a powerful and mature PHP class to manage uploaded files, and manipulate images in many ways. The script is available under a GPL license.
more info about the class
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$handle->image_ratio_no_zoom_in = true;
As per the doc:
"image_ratio_no_zoom_in same as image_ratio, but won't resize if the source image is smaller than image_x x image_y (default: false)"