fopen and load balancing

Hi, We had an issue with the code of a junior php developer that used fopen to load images using the url of the companies website that is load balanced. We could not the detect the problem in dev and test because the dev and test servers are not load balanced. I know that he could load the images using just the filesystem, but I am curious to know why it failed and if the load balancer is really the source of the problem or it is a limitation on the function itself. Thank you