PHP | SplDoublyLinkedList valid() Function

<?php
 
// Declare an empty SplDoublyLinkedList 
$list = new SplDoublyLinkedList();
 
// Use SplDoublyLinkedList::push() function to  
// add elements to the SplDoublyLinkedList 
$list->push(1); 
$list->push(2); 
$list->push(3); 
$list->push(8); 
$list->push(5); 

$list->rewind();

for($i = 0; $i < 5; $i++) {
    if($list->valid())
        echo $list->current() . " ";
    
    $list->next();
}

?>

<?php
 
// Declare an empty SplDoublyLinkedList 
$list = new SplDoublyLinkedList();
 
// Use SplDoublyLinkedList::add() function
// to add the element into the list
$list->add(0, "Welcome"); 
$list->add(1, "to"); 
$list->add(2, "GeeksforGeeks"); 
$list->add(3, "A"); 
$list->add(4, 'Computer'); 
$list->add(5, "Science"); 
$list->add(6, 'Portal'); 
 
$list->rewind();

for($i = 0; $i < 7; $i++) {
    if($list->valid())
        echo $list->current() . " ";
    
    $list->next();
}

?>