Description:
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As a user rightly pointed out a couple of years ago (http://www.php.net/manual/en/function.bcpowmod.php#73406), the statement is incorrectly written:

"Note: Because this method uses the modulus operation, non-natural numbers may give unexpected results. A natural number is any positive non-zero integer. "

Which is directly from the documentation, meanwhile it is else where defined as:

"In mathematics, there are two conventions for the set of natural numbers: it is either the set of positive integers {1, 2, 3, ...} according to the traditional definition; or the set of non-negative integers {0, 1, 2, ...} according to a definition first appearing in the nineteenth century."

@see http://en.wikipedia.org/wiki/Natural_number

Perhaps it should read:

"Note: This method uses the modulus operation, which may produce unexpected results if supplied with anything other than a non-zero positive integer."