odlist_reverse.php

<?php

class ODList
{
    /** string $name */
    public $name;

    /** ODList|null $next */
    public $next;

    public function __construct(string $name, ODList $next = null)
    {
        $this->name = $name;
        $this->next = $next;
    }

    public function __toString()
    {
        return $this->next ? "{$this->name} -> {$this->next}" : $this->name;
    }
}

$list = new ODList('a', new ODList('b', new ODList('c', new ODList('d'))));

echo (string)$list.PHP_EOL;

for ($current = $list, $prev = null; $current !== null; $current = $next) {
    $next = $current->next;
    $current->next = $prev;
    $prev = $current;
}

echo (string)$prev.PHP_EOL;