Please give me solution of this error

<?php

class database {

	public $sql = '';
	public $query = '';
	public $con;
	function __construct(){
		self::db();
	}	
	
	static function db()
	{	
		
		if($_SERVER['HTTP_HOST'] == 'localhost'){
	
			$host = 'localhost';
			$username = 'root';
			$pass = '';
			$db = 'alltoitskin';
		
		}else{
			
			$host = 'alltoitskin.db.7649825.hostedresource.com';
			$username = 'alltoitskin';
			$pass = 'Skinlogica123';
			$db ='alltoitskin';
		}
		
		$con=mysqli_connect($host, $username ,$pass)or die("mysql not connected");				
		mysqli_select_db($con,$db) or die('Invalid database selected');


		
	}
	
	function result_array($query){
			
			$this->query = $query;
			self::db();
									
		$result = array();
		$Q=mysqli_query($con,$query);
		
		if($Q && mysqli_num_rows($Q) > 0){
			while($tmp = mysqli_fetch_assoc($Q)){
				$result[] = $tmp;
			}
		}		
		return $result;
		
	}
	
	
	function result_row($query){
			
			$this->query = $query;
			self::db();		
		
		$result = array();
		//$Q=mysql_query($query);
		$Q=mysqli_query($con,$query);
		
		if($Q && mysqli_num_rows($Q) > 0){
			$tmp = mysqli_fetch_assoc($Q);
			return $tmp;			
		}
		
		return $result;
		
	}
	
	
	function insert($data = array(), $table = ''){		
		$t = sizeof($data);		
		if(empty($data) || $table == ''){
			return false;
		}
			
		$this->sql = 'INSERT INTO '.$table.' SET ' ;
		$i = 1;
		foreach($data as $k => $v){
			$this->sql .= $k." = '".$v."' ";
			
			if($i < $t ){
				$this->sql .= ' , ';
			}
			$i++;			
		}
		
		//echo $this->sql;
		
		$Q = mysql_query($this->sql);
				
		if($Q){			
			return mysql_insert_id();
		}
		
		return false;			
	}
	
	
	function num_rows($query){
	
		$this->query = $query;
		self::db();
		
		$result = array();
		$Q=mysql_query($query);
		
		if($Q){
			return mysql_num_rows($Q);
		}
		return 0;
		
	}
	
	function update($query){
		
		$this->query = $query;
		$Q = 	mysql_query($query);
		
		if($Q && mysql_affected_rows() > 0 ){
			return true;
		}
		
		return false;
	}
	
	function last_query(){
		return $this->query;
	}
	

}
?>

Notice: Undefined variable: con in C:\xampp\htdocs\grip-engineering\classes\database.php on line 62

Warning: mysqli_query() expects parameter 1 to be mysqli, null given in C:\xampp\htdocs\grip-engineering\classes\database.php on line 62