Because you're trying to store a string literal into a variable, then use that variable as an operator, rather than using the string literal as the symbol for the operator.
The crux of it is that:
$caryear $notifycaryearop $notifycaryear
is simply wrong syntax.
Even if it were accepted, it would become:
'1996 >= 2000'
not
1996 >= 2000
PHP strings can be interpreted as integers, but the same doesnt apply where operators instead of numbers are concerned.