diplaying checkborder of pictures in templatephp

Hello always following my idea of a checkerboard php 6 columns 4 rows to display random images, not wanting to have something too heavy-level display, processing time php I do this, only how to integrate a meter The idea is always to display images at random and that a click is enough to see the image containing:

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Only this was not correct or the idea of the function used microtime was interesting to display random.