Solution.php

# Definition for singly-linked list.
# class ListNode {
#     public $val;
#     public $next;
#     public function __construct($val = 0, $next = null)
#     {
#         $this->val = $val;
#         $this->next = $next;
#     }
# }

class Solution {
    /**
     * @param ListNode $list1
     * @param ListNode $list2
     * @return ListNode
     */

    function mergeTwoLists($list1, $list2) {
        $dummy = new ListNode(0);
        $current = $dummy;

        while ($list1 != null && $list2 != null) {
            if ($list1->val <= $list2->val) {
                $current->next = $list1;
                $list1 = $list1->next;
            } else {
                $current->next = $list2;
                $list2 = $list2->next;
            }
            $current = $current->next;
        }
        if ($list1 != null) {
            $current->next = $list1;
        } elseif ($list2 != null) {
            $current->next = $list2;
        }
        return $dummy->next;
    }
}