Description:
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Expresions like (NULL == 0) or (NULL == '') evaluate to TRUE. I doubt whether it is correct, but unambiguously it is inconvenient. If a variable $Var = 0 or $Var = '' then empty($Var) returns TRUE, while isset($Var) returns 
TRUE too. From the other side when $Var = NULL isset($Var) returns FALSE. From this point empty string with 0 and NULL are not the same. Moreover manual says
"Also note that a NULL byte ("\0") is not equivalent to the PHP NULL constant." So why NULL is equal to 0 and '' then?
NULL means that variable has no value in php context or in other words is unset. So, I guess that check like $Var == NULL must has the same meaning like !isset($Var).