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

In this method, first node in the 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 doubly linked list is checked for null value. If the head is not null and the value stored in it is equal to the key, make head next as head, delete previous head, and adjust previous of 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 3-step process.

public function pop_first($key) {     
  $temp = $this->head;
  //1. check if the head is not null
  if($temp != null) {
    
    //2. if head is not null and value stored at head
    //   is equal to the key, make head next as head,
    //   delete previous head, adjust previous of head
    if($temp->data == $key) {
      $nodeToDelete = $this->head;
      $this->head = $this->head->next;
      $nodeToDelete = null;
      if ($this->head != null)
        $this->head->prev = null;
    } else {

      //3. Else, traverse to the node previous to the 
      //   node with value equal to key, and adjust links 
      while($temp->next != null) {
        if($temp->next->data == $key) {
          $nodeToDelete = $temp->next;
          $temp->next = $temp->next->next;
          if($temp->next != null)
            $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 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;
    } else {
      $temp = new Node();
      $temp = $this->head;
      while($temp->next != null) {
        $temp = $temp->next;
      }
      $temp->next = $newNode;
      $newNode->prev = $temp;
    }    
  }

  //Delete first node by key
  public function pop_first($key) {     
    $temp = $this->head;
    if($temp != null) {
      if($temp->data == $key) {
        $nodeToDelete = $this->head;
        $this->head = $this->head->next;
        $nodeToDelete = null;
        if ($this->head != null)
          $this->head->prev = null;
      } else {
        while($temp->next != null) {
          if($temp->next->data == $key) {
            $nodeToDelete = $temp->next;
            $temp->next = $temp->next->next;
            if($temp->next != null)
              $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($temp != null) {
        echo $temp->data." ";
        $temp = $temp->next;
      }
      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 the 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