Define Constants can not be redeclared in php

while working on an email template script i had a situation like this yesterday.
i had an include file where
define (‘HTML_EMAIL’ , ‘false’) was defined.
on another file i wanted to change the value HTML_EMAIL to true.
the first thought came to mind is to redeclare the constant and i tried like below,
define (‘HTML_EMAIL’ , ‘true’) on another file // not correct
when i called for the value then HTML_EMAIL value was false as old value.
it happened because i tried to assign value to a CONSTANT ( as it is already declared on include file).
well if value can be changed then it can not be a constant.
so defined constants can not be redeclared in php again.
try to use variables on similar situations.

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