How to permanently typecast class properties in PHP?

I have been reading the "Visibility" section of the PHP manual, and in the first comment, someone mentions:

OUTSIDE CODE can cast Item properties to any other PHP types (booleans, integers, floats, strings, arrays, and objects etc.) -- another huge mistake.

Consider this example:

class base {
    public $foo = 1;
}

$first = new base();

(string)$first->foo; //I thought just this expression would typecast
var_dump($first->foo); //but I found it still is int


$first->foo = (string)$first->foo;    
var_dump($first->foo); //ok so public props can be typecasted

Is it just with the protected and private properties that we cannot change their type from outside? Or does that apply to public properties, too?

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