Why does PHP's session not include a value to determine whether it is null and a warning solution appears?

Why is the PHP session NULL? SESSION_START (); if ($ _ SESSION ['AA'] = NULL) {echo & quot; null & quot;} else {echo & quot; non-empty & quot;} determines whether the result is feasible? Do you want to attach an initial value? However, why does the PHP session not include a value to determine whether it is null? a warning is reported.
SESSION_START ();
If ($ _ SESSION ['AA'] = NULL ){
Echo "null ";
} Else {
Echo "not empty ";
}

Is this true? Do you want to attach an initial value? However, if I jump to another page and return, the initial value will be cleared. how can I solve this problem?

------ Solution --------------------
Use isset or empty
Specifically, do you want to determine whether the variable exists or whether the content is null?
Will it be cleared when I jump to another page? Paste all codes