PHP - Delete first node by key of the Circular Doubly Linked List

In this method, first node in the circular doubly linked list with specified key (value) is deleted. For example - if the given list is 10->20->30->10->20 and the first occurrence of 20 is deleted, the list becomes 10->30->10->20.

First, the head of the linked list is checked for null value. If the head is not null, the value stored in it is equal to the key, and head is the only element in the list then make the head as null. If the head is not null, the value stored in it is equal to the key, the list contains more than one element then make head next as head and update the link of last element of the list with the new head. Else, traverse to the node previous to the node with value equal to key, and adjust links.

The function pop_first is created for this purpose. It is a 4-step process.

public function pop_first($key) {
  
  //1. if head is not null, create three nodes- temp,
  //   nodeToDelete and lastNode - to traverse and to 
  //   track the last node and node to delete  
  if($this->head != null) {
    $temp = $this->head;
    $nodeToDelete = $this->head;
    $lastNode = new Node();

    //2. if the value store at head is the key and head
    //   is the only element in the list, make it null
    if($temp->data == $key) {
      if($temp->next == $this->head) {
        $this->head = null;
      } else {

        //3. if the value store at head is the key and list
        //   contains more than 1 elements, adjust links
        $lastNode = $this->head->prev;
        $this->head = $this->head->next;
        $lastNode->next = $this->head;
        $this->head->prev = $lastNode;
        $nodeToDelete = null; 
      }
    } else {
      
      //4. Else, traverse to the node previous to the 
      //   node with value equal to key, and adjust links 
      while($temp->next != $this->head) {
        if($temp->next->data == $key) {
          $nodeToDelete = $temp->next;
          $temp->next = $temp->next->next;
          $temp->next->prev = $temp;
          $nodeToDelete = null;
          break; 
        }
        $temp = $temp->next;
      }
    }
  }
}

The below is a complete program that uses above discussed concept to delete first occurrence of the specified key (if exists) of the circular doubly linked list.

<?php
//node structure
class Node {
  public $data;
  public $next;
  public $prev;
}

class LinkedList {
  public $head;

  public function __construct(){
    $this->head = null;
  }
  
  //Add new element at the end of the list
  public function push_back($newElement) {
    $newNode = new Node();
    $newNode->data = $newElement;
    $newNode->next = null; 
    $newNode->prev = null;
    if($this->head == null) {
      $this->head = $newNode;
      $newNode->next = $this->head;
    } else {
      $temp = new Node();
      $temp = $this->head;
      while($temp->next !== $this->head) {
        $temp = $temp->next;
      }
      $temp->next = $newNode;
      $newNode->next = $this->head;
      $newNode->prev = $temp;
      $this->head->prev = $newNode;
    }    
  }

  //Delete first node by key
  public function pop_first($key) { 
    if($this->head != null) {
      $temp = $this->head;
      $nodeToDelete = $this->head;
      $lastNode = new Node();
      if($temp->data == $key) {
        if($temp->next == $this->head) {
          $this->head = null;
        } else {
          $lastNode = $this->head->prev;
          $this->head = $this->head->next;
          $lastNode->next = $this->head;
          $this->head->prev = $lastNode;
          $nodeToDelete = null; 
        }
      } else {
        while($temp->next != $this->head) {
          if($temp->next->data == $key) {
            $nodeToDelete = $temp->next;
            $temp->next = $temp->next->next;
            $temp->next->prev = $temp;
            $nodeToDelete = null;
            break; 
          }
          $temp = $temp->next;
        }
      }
    }
  }

  //display the content of the list
  public function PrintList() {
    $temp = new Node();
    $temp = $this->head;
    if($temp != null) {
      echo "The list contains: ";
      while(true) {
        echo $temp->data." ";
        $temp = $temp->next;
        if($temp == $this->head)
          break;        
      }
      echo "\n";
    } else {
      echo "The list is empty.\n";
    }
  }    
};

// test the code  
$MyList = new LinkedList();

//Add five elements at the end of the list.
$MyList->push_back(10);
$MyList->push_back(20);
$MyList->push_back(30);
$MyList->push_back(10);
$MyList->push_back(20);
$MyList->PrintList();

//Delete first occurrence of 20
$MyList->pop_first(20);
$MyList->PrintList();
?>

The above code will give the following output:

The list contains: 10 20 30 10 20 
The list contains: 10 30 10 20