Please what's wrong with my codes

class DatabaseManager
  {
	
		//variable to hold connection object.
		protected static $connection;
	
		//constructor - called on object creation
		private function __construct() {
	
			try {
				// assign new PDO object to db variable
				self::$connection = new PDO( 'mysql:host=localhost; dbname=mydb', 'username', 'pass' );
				//sets encoding to UTF-8
				#self::$connection->exec("SET CHARACTER SET utf8");
				//Enable error mode - use try/catch to catch error
				self::$connection->setAttribute( PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION );
			}
			catch (PDOException $e) {
				//Output error - would normally log this to error file rather than output to user.
				echo "Connection Error: " . $e->getMessage();
			}
	
		}
	
		// get connection function. Static method - accessible without instantiation
		public static function getConnection() {
	
			//Guarantees single instance, if no connection object exists then create one.
			if (!self::$connection) {
				//new connection object.
				new DatabaseManager();
			}
	
			//return connection.
			return self::$connection;
		}
	

public function insertAsset()
	{
		// Does the Asset object already have an ID?
	if ( !is_null( $this->id ) ) trigger_error ( "AssetManager::insertAsset(): Attempt to insert an Asset object that already has its ID property set (to $this->id).", E_USER_ERROR );
		
		// Insert the Asset
		$conn = DatabaseManager::getConnection();
		$sql = "INSERT INTO table_asset ( productName, productSerialNumber, purchaserOrSupplier, supplierPhoneNo, status, purchaseDate, 
	 departmentOfUsage, quantityPurchased, costPerUnit, remark) VALUES ( :productName, :productSerialNumber, 
		:purchaserOrSupplier, :supplierPhoneNo, :status, FROM_UNIXTIME(:purchaseDate), 
	 :departmentOfUsage, :quantityPurchased, :costPerUnit, :remark )";
		$st = $conn->prepare ( $sql );
		$st->bindValue( ":productName", self::productName, PDO::PARAM_STR );
		$st->bindValue( ":productSerialNumber", self::productSerialNumber, PDO::PARAM_STR );
		$st->bindValue( ":purchaserOrSupplier", self::purchaserOrSupplier, PDO::PARAM_STR );
		$st->bindValue( ":supplierPhoneNo", self::supplierPhoneNo, PDO::PARAM_INT );
		$st->bindValue( ":status", self::status, PDO::PARAM_STR );
		$st->bindValue( ":purchaseDate", self::purchaseDate, PDO::PARAM_INT );
		$st->bindValue( ":departmentOfUsage", self::departmentOfUsage, PDO::PARAM_STR );
		$st->bindValue( ":quantityPurchased", self::quantityPurchased, PDO::PARAM_INT );
		$st->bindValue( ":costPerUnit", self::costPerUnit, PDO::PARAM_INT );
		$st->bindValue( ":remark", self::remark, PDO::PARAM_STR );
		$st->execute();
		$this->id = $conn->lastInsertId();
		$conn = null;
		}
		

( ! ) Fatal error: Using $this when not in object context in C:\wamp\www\ec\classes\AssetManager.php on line 197
Call Stack
#	Time	Memory	Function	Location
1	0.0013	371032	{main}( )	..\assetProcessor.php:0
2	0.0191	470720	AssetManager::insertAsset( )	..\assetProcessor.php:11