#3 Variable substitution is different

[Jim Hart]

If a variable is set, not to the contents of another
variable, but to the variable itself, by escaping the $
in the assignment, when that variable is later echoed,
Apache displays the $ followed by the variable name.
exssi displays the value from the second variable.

[Jim Hart]

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exssi's current variable substitution is fully recursive.
That is, it continues to replace variables with values as
long as there are any variables left in the expression being
analyzed, no matter how many levels of nesting occur.
Apparently, Apache only does a single pass.

We should add a configuration option for "Apache
compatibility" which disables any exssi enhancements that
would make existing XSSI web pages "break".