Sorry, totally wrong, it should be: 
'ab' = (1*(26**1)) + (2*(26**0))

'ab' -- = (1*(26**1)) + ((2*(26**0)) - (1*(26*0)))

        = (1*(26**1)) + (1*(26**0))

        = 'aa'

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But ideally a string containing only alphanumerics with an alphabet in upper and lower case could be handled as a Base 62 number which would also allow decrement, because the question with the above is how does it handle numerics and upper case. Base 62 would solve that.

In reply to Re: Re: Re: $a++ allowed by $a-- is not ! why? by Anonymous Monk
in thread $a++ allowed by $a-- is not ! why? by abhishes

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