How can I undo/reverse an XOR with PHP?

I'm a bit confused by XOR, conceptually. I have an light encryption function I need to decrypt, an开发者_开发技巧d I'm not sure how to get it working correctly.

If my value was originally generated by:

$val = dechex($seed^$id);

Then, elsewhere, I have the corresponding $val and $seed, how can I generate the $id?

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XOR is its own inverse, so you can just XOR $val by $seed again and get $id. You might need to run hexdec on $val first, though.