If it's no coincidence that 15 gets mapped to 1, 20 to 2, etc, i.e. all multiples of some number and the corresponding values increasing, then you could view the problem a little differently. There's a linear relation between (15, 1), (20, 2), (25, 3),... which can be expressed by y = x/5 - 2. All values you give satisfy this equation as can be verified easily.

Now we can compute the corresponding number for $x simply as:

  $number = ceil($x/5 - 2);
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Hope this helps, -gjb-