PHP Error: Function name must be a string in

Solution:

 if(!isset($_session)) {
 $session_start();}

$session_start() should be session_start(). $ is for variables, session_start() is a function, not a variable, thus, $ is not required.

Also, it must be capitalised as it’s a superglobal array. (Credits to @AkshayPrakash for pointing out)

 if(!isset($_SESSION)) {
 session_start();}