Strange Problem In Php Oop On Localhost

I am trying to get into the Object Oriented Programming (OOP) in the PHP. For this I have installed a local apache server to test my work and experiment with my scripting techniques. Until today everything was going fine and my learning speed was good. But I have come to face a problem to which I have no answer.

I am trying to create a user class for a test project. The code of the user class is as follows:

<?phprequire_once("database.php");class User {public $id;public $username;public $password;public $first_name;public $last_name;public static function find_all() {  return self::find_by_sql("select * from users");}public static function find_by_id($id=0) {  global $database;  $result_array=self::find_by_sql("Select * from users where id={$id} limit 1");  if (!empty($result_array)) {   return array_shift($result_array);  } else {   return FALSE;  }}public function find_by_sql($sql="") {  global $database;  $result_set=$database->query($sql);  $object_array=array();  while ($row=$database->fetch_array($result_set)) {   $object_array[]=self::instantiate($row);  }   return $object_array;}public function full_name() {  if (isset($this->first_name)&& isset($this->last_name)) {  return $this->first_name." ".$this->lastname;   } else {    return "";   }}private static function instantiate($record) {  $object=new self;  $object->id=$record['id'];  $object->username=$record['username'];  $object->passwrod=$record['password'];  $object->first_name=$record['first_name'];  $object->last_name=$record['last_name'];  return $object;}}?>

As you can see in the code, I have created a User class with few attributes and methods. The code is clean and there is no problem in it. On the index page, I have the following code:

$user=User::find_by_id(1);echo $user->full_name();

Theoretically, the code should return the full name of a user but instead when I run the code, I get the following error:

( ! ) Fatal error: Call to a member function full_name() on a non-object in C:\wamp\www\imagepro\public\index.php on line 8Call Stack# Time Memory Function Location1 0.0014 669408 {main}( ) ..\index.php:0

And I don't understand why I am getting this error. It says that I am making a call to an object function from a non-object. But as per the code in the instantiate method of the user class, the object should get automatically instantiated. I have spend more than an hour just to figure out any mistake in the code but I am unable to find it. So the last guess that is left with me is that there are some settings that I need to change in the php.ini file on my localhost.

Any help from the experts is appreciated.
Thanks in advance

As I understand just by looking, the variable $user is not an object, it has the value of the method find_by_id();