PHP variable scope - is the variable used anywhere?

Question

I am trying to read some php code, but can't understand how the variable $predicate here can be used:

private function getRelatedItems() {
    $predicate = Zotero_Relations::$relatedItemPredicate;

    $relations = $this->getRelations();
    if (empty($relations->$predicate)) {
        return [];
    }

    $relatedItemURIs = is_string($relations->$predicate)
        ? [$relations->$predicate]
        : $relations->$predicate;

    // Pull out object values from related-item relations, turn into items, and pull out keys
    $keys = [];
    foreach ($relatedItemURIs as $relatedItemURI) {
        $item = Zotero_URI::getURIItem($relatedItemURI);
        if ($item) {
            $keys[] = $item->key;
        }
    }
    return $keys;
}

As far as I can see $predicate is assigned a value that is never used. But I guess I am misunderstanding the scope somehow, or?

Answer 1

The variable is used to access a property in $relations variable.

Here is a simplified usage example of variable property access which should clarify the usage for you:

$property = 'firstName';
$data = (object) array(
    'firstName' => 'Justin',
    'lastName' => 'Case',
);

echo $data->$property; //Rquivalent to $data->firstName, which eEchoes 'Justin'

Additionally, it works for functions, and class methods, too:

class Foo
{
     public function bar()
     {
         echo 'Hello, world!';
     }
}

$foo = new Foo();
$method = 'bar';

$foo->$method(); //Equivalent to $foo->bar(); Displays 'Hello, world!'


$function = 'strlen';

echo $function('Hello, world!'); //Is equivalent to calling strlen('Hello, world!');

It's pretty neat!

Answer 2

The $predicate variable holds the name of an attribute on the $relations object. E.g:

$relations->$predicate

If $predicate is set to foo then PHP sees the line as:

$relations->foo