PHP - Delete the last node of the Linked List

In this method, the last node of the linked list is deleted. For example - if the given list is 10->20->30->40 and the last node is deleted, the list becomes 10->20->30.

Deleting the last node of the Linked List involves checking the head for empty. If it is not empty, then check the head next for empty. If the head next is empty, then release the head, else traverse to the second last node of the list. Then, link the next of second last node to NULL and delete the last node.

The function pop_back is created for this purpose. It is a 3-step process.

public function pop_back() {
  if($this->head != null) {
    
    //1. if head in not null and next of head
    //   is null, release the head
    if($this->head->next == null) {
      $this->head = null;
    } else {
      
      //2. Else, traverse to the second last 
      //   element of the list
      $temp = new Node();
      $temp = $this->head;
      while($temp->next->next != null)
        $temp = $temp->next;
      
      //3. Change the next of the second 
      //   last node to null and delete the
      //   last node
      $lastNode = $temp->next;
      $temp->next = null; 
      $lastNode = null;
    }
  }
}

The below is a complete program that uses above discussed concept of deleting the last node of the linked list.

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

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;
    $newNode->next = null; 
    if($this->head == null) {
      //for first element in the list
      $this->head = $newNode;
    } else {
      $temp = new Node();
      $temp = $this->head;
      while($temp->next != null) {
        $temp = $temp->next;
      }
      $temp->next = $newNode;
    }    
  }

  //Delete last node of the list
  public function pop_back() {
    if($this->head != null) {
      if($this->head->next == null) {
        $this->head = null;
      } else {
        $temp = new Node();
        $temp = $this->head;
        while($temp->next->next != null)
          $temp = $temp->next;
        $lastNode = $temp->next;
        $temp->next = null; 
        $lastNode = null;
      }
    }
  }

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

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

//Add four elements in the list.
$MyList->push_back(10);
$MyList->push_back(20);
$MyList->push_back(30);
$MyList->push_back(40);
$MyList->PrintList();

//Delete the last node
$MyList->pop_back();
$MyList->PrintList(); 
?>

The above code will give the following output:

The list contains: 10 20 30 40
The list contains: 10 20 30