Can't use function return value in write context

[sonicfusion]

if(strlen($_POST["room"]) > 1 && strlen($_POST["month"]) >= 1 && strlen($_POST["year"]) = 4){
include_once("calendar.inc.php");
calendar($_POST["room"], $_POST["month"], $_POST["year"]);
}

i'm getting an error:
Fatal error: Can't use function return value in write context in......

What does this mean, and is there a solution?

thank you all

[rincewind456]

This

&& strlen($_POST["year"]) = 4){

should be

&& strlen($_POST["year"]) == 4){

That may help.