Simple linked list in PHP

This code demonstrate the implementation of linked list using PHP. 

<?  class ListItem{

 var $key;

 var $next;

 var $prev;

   function ListItem($key=NULL){

  $this->key=$key;

  $this->next=NULL;

  $this->prev=NULL;

 }

}

 class LinkedList{

 var $head;

   function LinkedList(){

  $this->head=NULL;

 }

 function ListInsert(&$x){  

  $x->next = &$this->head;

  if(!is_null($this->head))$this->head->prev=&$x;

  $this->head=&$x;

  $this->head->prev=NULL;

 }

  function ListSearch($k){

  $x=&$this->head;

  while(!is_null($x) && $x->key!=$k){

   $x=&$x->next;

  }

 return $x;

 }

  function ListDelete(&$x){

  if (!is_null($x->prev)) $x->prev->next=&$x->next;

  else $this->head=&$x->next;

  if (!is_null($x->next)) $x->next->prev=&$x->prev;

 }

   function Listshow(){

  $x=&$this->head;

  $b=1;

  while ($b){

   echo 'key:'.$x->key."n";

   if(is_null($x->next))$b=0;

   $x=&$x->next;

  } 

  }

 }

?>

Notes: I have difficulties in writing codes in blogsopt. The solution is, write the post in google docs, and send this article to the blogspot. Helluva job Kerul...