A doubly linked list is a linear data structure, in which the elements are stored in the form of a node. Each node contains three sub-elements. A data part that stores the value of the element, the previous part that stores the link to the previous node, and the next part that stores the link to the next node as shown in the below image:

The first node also known as HEAD is always used as a reference to traverse the list. The previous of head node and next of last node points to NULL. A doubly linked list can be visualized as a chain of nodes, where every node points to previous and next node.

Representation:

In PHP, doubly linked list can be represented as a class and a Node as a separate class. The LinkedList class contains a reference of Node class type.

```//node structure
class Node {
public \$data;
public \$next;
public \$prev;
}

//constructor to create an empty LinkedList
public function __construct(){
}
};
```

Let us create a simple doubly linked list which contains three data nodes.

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

//constructor to create an empty LinkedList
public function __construct(){
}
};

// test the code

\$first = new Node();
\$first->data = 10;
\$first->next = null;
\$first->prev = null;

\$second = new Node();
\$second->data = 20;
\$second->next = null;
\$second->prev = \$first;
\$first->next = \$second;

\$third = new Node();
\$third->data = 30;
\$third->next = null;
\$third->prev = \$second;
\$second->next = \$third;
?>
```

A doubly linked list can be traversed using a temp node. Keep on moving the temp node to the next one and displaying its content. At the end of the list, the temp node will become NULL.

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

//constructor to create an empty LinkedList
public function __construct(){
}

//display the content of the list
public function PrintList() {
\$temp = new Node();
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

\$first = new Node();
\$first->data = 10;
\$first->next = null;
\$first->prev = null;

\$second = new Node();
\$second->data = 20;
\$second->next = null;
\$second->prev = \$first;
\$first->next = \$second;

\$third = new Node();
\$third->data = 30;
\$third->next = null;
\$third->prev = \$second;
\$second->next = \$third;

//print the content of list
\$MyList->PrintList();
?>
```

The above code will give the following output:

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

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