error_reporting(E_ALL); //Show all errors, warning, notifications - good practice! 

if(isset($_GET['no']) && $_GET['no'] != ""){ 
// This checks if $_GET['no'] is set - as in it exists, but also checks if it contains anything. If it doesn't we execute the else
	$no = $_GET["no"];
	echo 'test1';
} else {
	$no = '405';
	echo 'test2';
}

var_dump($_GET['no']); //This line will display detailed information of what $_GET['no'] has set in it, very useful for debugging