How to differentiate a value is zero or NULL in php?

I have a variable e.g $moon

In some cases its value is

$moon=0;

In other cases its value is

$moon=NULL;

Now in if clause how can i differentiate between these values?

My code is

if($moon==0 && !empty($moon)){
    echo "its zero";        
}else{
    echo "its null";
}

it always shows "its zero"

I also tried

if($moon==0 && $moon!=NULL){
    echo "its zero";        
}else{
    echo "its null";
}