Returns the value at the specified key in the current
public function at( int $key, ): mixed;
If the key is not present, an exception is thrown. This essentially means
if you specify a key other than 0 or 1, you will get an exception. If you
don't want an exception to be thrown, use
$v = $p->at($k)" is semantically equivalent to
$v = $p[$k].
int $key- the key from which to retrieve the value.
mixed- The value at the specified key; or an exception if the key does not exist.