在preg_replace中使用/ e标志时，PHP不评估只读属性

Question

think we have this script :

class String {
protected $text = null;
public function __construct($text){
    $this->text = $text;
}

public function __get($var){ return $this->$var; }

public function replace($search, $replace, $limit = -1, $ignoreCase = false){
    extract($GLOBALS, EXTR_REFS);
    return preg_replace($search, $replace, $this->text, $limit, $count);
}
}

class Setting {
private $active = "not active";

public function __get($var){
    return $this->$var;
}
}

$s = new Setting;

function replace(){
$string = new String('System is [var:$s->active]');
echo $string->replace('/\[var:([^\]]+)\]/ie', 'isset(\1)? \1: "";');
}

replace();

now $active property will not evaluate is it a bug or i should do something special ?

SOLVED
with a lot of thanks to dear Artefacto.
the problem was solved

i should implement __isset to use isset function for readonly property

Answer 1

Have to implement __isset function for String class when want to use isset()

class Setting {
    public function __isset($var){
        return isset($this->$var);
    }
}