class DB{
    private $dbHost     = "localhost";
    private $dbUsername = "root";
    private $dbPassword = "";
    private $dbName     = "computer network";

public function __construct(){
    if(!isset($this->db)){
        // Connect to the database
        $conn = new mysqli($this->dbHost, $this->dbUsername, $this->dbPassword, $this->dbName);
        if($conn->connect_error){
            die("Failed to connect with MySQL: " . $conn->connect_error);
        }else{
            $this->db = $conn;
        }
    }
}

/*
 * Returns rows from the database based on the conditions
 * @param string name of the table
 * @param array select, where, order_by, limit and return_type conditions
 */
public function getRows($table,$conditions = array()){
    $sql = 'SELECT ';
    $sql .= array_key_exists("select",$conditions)?$conditions['select']:'*';
    $sql .= ' FROM '.$table;


    if(array_key_exists("where",$conditions)){
        $sql .= ' WHERE ';
        $i = 0;
        foreach($conditions['where'] as $key => $value){
            $pre = ($i > 0)?' AND ':'';
            $sql .= $pre.$key." != '".$value."'";
            $i++;
        }
    }

    if(array_key_exists("order_by",$conditions)){
        $sql .= ' ORDER BY '.$conditions['order_by'];
    }

    if(array_key_exists("start",$conditions) && array_key_exists("limit",$conditions)){
        $sql .= ' LIMIT '.$conditions['start'].','.$conditions['limit'];
    }elseif(!array_key_exists("start",$conditions) && array_key_exists("limit",$conditions)){
        $sql .= ' LIMIT '.$conditions['limit'];
    }

    $result = $this->db->query($sql);

    if(array_key_exists("return_type",$conditions) && $conditions['return_type'] != 'all'){
        switch($conditions['return_type']){
            case 'count':
                $data = $result->num_rows;
                break;
            case 'single':
                $data = $result->fetch_assoc();
                break;
            default:
                $data = '';
        }

}else{
        if($result->num_rows > 0){
            while($row = $result->fetch_assoc()){
                $data[] = $row;
            }
        }
    }
    return !empty($data)?$data:false;
}

/*
 * Insert data into the database
 * @param string name of the table
 * @param array the data for inserting into the table
 */
public function insert($table,$data){
    if(!empty($data) && is_array($data)){
        $columns = '';
        $values  = '';
        $i = 0;
        if(!array_key_exists('created',$data)){
            $data['created'] = date("Y-m-d H:i:s");
        }
        if(!array_key_exists('modified',$data)){
            $data['modified'] = date("Y-m-d H:i:s");
        }
        foreach($data as $key=>$val){
            $pre = ($i > 0)?', ':'';
            $columns .= $pre.$key;
            $values  .= $pre."'".$val."'";
            $i++;
        }
        $query = "INSERT INTO ".$table." (".$columns.") VALUES (".$values.")";
        $insert = $this->db->query($query);
        return $insert?$this->db->insert_id:false;
    }else{
        return false;
    }
}

/*
 * Update data into the database
 * @param string name of the table
 * @param array the data for updating into the table
 * @param array where condition on updating data
 */
public function update($table,$data,$conditions){
    if(!empty($data) && is_array($data)){
        $colvalSet = '';
        $whereSql = '';
        $i = 0;
        if(!array_key_exists('modified',$data)){
            $data['modified'] = date("Y-m-d H:i:s");
        }
        foreach($data as $key=>$val){
            $pre = ($i > 0)?', ':'';
            $colvalSet .= $pre.$key."='".$val."'";
            $i++;
        }
        if(!empty($conditions)&& is_array($conditions)){
            $whereSql .= ' WHERE ';
            $i = 0;
            foreach($conditions as $key => $value){
                $pre = ($i > 0)?' AND ':'';
                $whereSql .= $pre.$key." = '".$value."'";
                $i++;
            }
        }
        $query = "UPDATE ".$table." SET ".$colvalSet.$whereSql;
        $update = $this->db->query($query);
        return $update?$this->db->affected_rows:false;
    }else{
        return false;
    }
}

/*
 * Delete data from the database
 * @param string name of the table
 * @param array where condition on deleting data
 */
public function delete($table,$conditions){
    $whereSql = '';
    if(!empty($conditions)&& is_array($conditions)){
        $whereSql .= ' WHERE ';
        $i = 0;
        foreach($conditions as $key => $value){
            $pre = ($i > 0)?' AND ':'';
            $whereSql .= $pre.$key." = '".$value."'";
            $i++;
        }
    }
    $query = "DELETE FROM ".$table.$whereSql;
    $delete = $this->db->query($query);
    return $delete?true:false;
}