Wrapping up the website task 5/7

If there ever was a code challenge that rubbed me up the wrong way, this would be it. here is the question:

"PalprimeChecker objects have a method called isPalprime(). This method does not receive any arguments. It returns true if the number property contains a palprime, and it returns false if the number property does not contain a palprime. (Tip: 17 is not a palprime.) Turn the second echo statement into a conditional that displays “is” or “is not” appropriately. The conditional should call the isPalprime method of the $checker object. If the isPalprime method returns true, then echo “is”; otherwise, echo “is not”."

require_once("class.palprimechecker.php");
$checker = new PalprimeChecker();

$checker -> number = 17;

echo "The number " . $number;

if($checker-> isPalprime()) {
    echo "is ";
} else {
    echo "is not ";
} else {
    echo "a palprime. ";
}

This is my best effort yet it doesn't pass.

1. What I want to know is how can "isPalprime" not receive arguments yet check if a number is a palprime?

2. how is "isPalprime" checking if $checker is a prime through "($checker->isPalprime)" because it looks like $checker is assigning a property.

3. How does "isPalprime" extract 17 from $checker since $number is just a property of the object $checker.

Any help would be appreciated.