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Baseline
isbn13("data")
takes an ISBN-10 format code as the data input and returns is in ISBN-13 format. Any dashes or other punctuation will be ignored. If the argument is not a valid ISBN code, the function returns an empty string. If such characters as in the source string and quotes are omitted, hyphen may be mis-parsed as minus signs giving a wrong result.
Consider a book with the ISBN-10 '1472268997' and the ISBN-13 '978-1472268990':
$MyString = isbn13("1472268997") gives the correct '9781472268990' but note, without any hyphens.
and
$MyString = isbn13(1472268997) gives the correct '9781472268990' but note without any hyphens..