Description:
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The documentation for strspn() seems to be incomplete. It donot explains the function behaviour with negative values of start and len argument.Without looking at the PHP source code one cannot confirm the behaviour of the function for negative start and len values.



Reproduce code:
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Following can be added to the documentation:

1. When fourth argument(len) is not passed
   a)if $start<0 and $start >= -(length of $str) then
     start=start+length of $str
   b)if $start<0 and start < -(length of $str A)
   c)By default, $len=length of $str

2. When all four arguments are passed to the function:
   a)if $start<0 and $start >= -(length of $str) then
     start=start+length of $str
   b)if $start<0 and start < -(length of $str A)
   c)if $len<0 and $len >= -(length of $str - $start) then
     len=len+(length of $str - $start)
   d)if $len<0 and $len < -(length of $str - $start) then
     len=0.