PHP error Can't use method return > value in write context

[P.A.T]

I am getting this error in a PHP class...\[quote\] Fatal error: Can't use method return value in write context in C:\webserver\htdocs\friendproject2\includes\classes\User.class.php on line 35\[/quote\]Here is the troubled part. \[code\]if(isset($this->session->get('user_id')) && $this->session->get('user_id') != ''){ //run code}\[/code\]This code is in my contrustor, is a value is not already set for $this->session->get('user_id') then it will return false instead of a Number. So as you can see I was hoping to check if this value is a number or not or not even set.Any help with fixing appreciated.