Re: Arbitrary mathematical relations, not just hashes

As far as languages with two-way relation go, there are many; perhaps the most prototypical is Lisp, in that either member of a pair within an alist can be used to look the pair up, with no extra function or second map definition required. But PHP has pretty good support, too, actually. If you have a 1-to-1 relation, you can use array_flip() to get a second array with the keys and values swapped in one go. If you have a 1-to-many, as in this case, you can use the optional search parameter to array_keys: >>> $color = array('apple' => 'red', 'ruby' => >>> 'red', 'banana' => 'yellow'); >>> array_keys($color, 'red') Array ( [0] => apple [1] => ruby )