PHP Fastest way to get image width and height

Well, i am currently updating my WordPress plugin. I faced a problem in the past to retrieve image width to determine whether a particular image is require to resize. The problem here is that checking a particular image height and width is expensive job. In order to not impact the loading time of the site using my plugin, i forfeited the capability of resizing and resize the image regardless of the size. The plugin works fine since the resizing is doing it on the fly. However, problems arise when a smaller image is being resize and this is not a desirable result. Therefore, i would like to see whether there is a solution exist that can easily solve my problem.