php filter_input parse error inside class

Configuration of a project in dev mode with WAMP.
PHP vers 5 and 7 are available.

Just trying to set the project root using filter_input. Could someone please explain why filter input for the protected and private vars inside the class reports a PARSE ERROR? However if used outside the class or inside a function of the class it works.

Is there a better way to do this so that it can be used globally? I find this is called a lot and would prefer to do it once.

$test = filter_input(INPUT_SERVER,'DOCUMENT_ROOT');
echo $test; //good

class FooBar{
    protected $_test = filter_input(INPUT_SERVER,'DOCUMENT_ROOT'); //bad - Parse error: syntax error, unexpected '(', expecting ',' or ';' 
    private $_test2 = filter_input(INPUT_SERVER,'DOCUMENT_ROOT'); //bad - Parse error: syntax error, unexpected '(', expecting ',' or ';' 
    function __construct() {
    }




    public function getProducts(){
        include_once 
 (filter_input(INPUT_SERVER,'DOCUMENT_ROOT').'/obj/user.php'); //good
   }
}

Solution

You can not directly assign a function return value to a property in the class definition.

This is because the function could return different return values, and the class is only a blueprint which you must instantiate as an object to use.

For the objects that are created from your class definition you can initialize any property in the constructor:

class FooBar {
    protected $var = null;
    private $var2 = null; 
    function __construct() {
        $this->var = func1();
        $this->var2 = func2();
    }
}
// no parse error

Despite of that, why do you use filter_input on an internal constant? You only need to filter input from the outside, i.e. GET/POST/SESSION content (user input), input read from files, from external APIs etc. But you don't need to use that on internal constants like the DOCUMENT_ROOT:

class FooBar {
    private $_docroot = $_SERVER['DOCUMENT_ROOT'];
}
// no parse error