I just started learning about MVC framework implementation. Essentially I have a DB class that wraps either PDO or MySQLi connection objects.

    class DB
{
    private static $_instance=null;
    private $_pdo, $_query, $_error=false, $_result, $_count=0, $_lastInsertId=null;
    
    private function __construct()
    {
        try
        {
            $this->_pdo=new PDO('mysql:host='.DB_HOST.';dbname='.DB_NAME, DB_USER, DB_PASSWORD);
            
        }
        catch(PDOException $e)
        {
            die($e->getMessage());
        }
    }
    
    public static function getInstance()
    {
        if(!isset(self::$_instance))
        {
            self::$_instance=new DB();
        }
        
        return self::$_instance;
    }

    public function first()
    {
        return (!empty($this->_result))?$this->_result[0]:[];
    }
    
    public function query($sql, $params=[])
    {
        $this->_error=false;
        if($this->_query=$this->_pdo->prepare($sql))
        {
            $x=1;
            if(count($params))
            {
                foreach($params as $param)
                {
                    $this->_query->bindValue($x, $param);
                    $x++;
                }
            }
            
            if($this->_query->execute())
            {
                $this->_result=$this->_query->fetchALL(PDO::FETCH_OBJ);
                $this->_count=$this->_query->rowCount();
                $this->_lastInsertId=$this->_pdo->lastInsertId();
            }
            else
            {
                $this->_error=true;
            }
        }
        return $this;
        
    }
    
    protected function _read($table, $params=[])
    {
        $conditionString='';
        $bind=[];
        $order='';
        $limit='';
        if(isset($params['conditions']))
        {
            if(is_array($params['conditions']))
            {
                foreach($params['conditions'] as $condition)
                {
                    $conditionString.=' '.$condition.' AND';
                }
                $conditionString=trim($conditionString);
                $conditionString=rtrim($conditionString, ' AND');
            }
            else
            {
                $conditionString=$params['conditions'];
            }
            if($conditionString!='')
            {
                $conditionString=' WHERE '.$conditionString;
            }
        }
        if(array_key_exists('bind', $params))
        {
            $bind=$params['bind'];
        }
        if(array_key_exists("order", $params ))
        {
            $order=' ORDER BY '.$params['order'];
        }
        if(array_key_exists("limit", $params ))
        {
            $limit=' LIMIT '.$params['limit'];
        }
        $sql="SELECT * FROM {$table} {$conditionString} {$order} {$limit}";
        if($this->query($sql, $bind))
        {
            if(!count($this->_result))
                return false;
            else
                return true;
        }
        return false;
    } 
    
    
    public function findFirst($table, $params)
    {
        if($this->_read($table, $params))
        {
            return $this->first();
        }
        else
            return false;
    }
    
    
     
}

There are some other methods, but I think they are not important.
So we have the _read() meathod that builds a Select query. The _read() method gets a table name and select query clauses as its arguments. The query() method that runs the query built by the _read() method. Also findFirst() method returns the first record in the result set of a select query.
The user_sessions table is of the following structure

     ______ ______________________________
    | id   | user_id| session |user_agent|

The session value and the user_agent values are stored in a cookie. Now when I try to run

      $db=new DB("some_table_name");
      $db->findFirst("user_sessions", ['conditions'=>'user_agent= ? AND session= ?',
            'bind'=>[Session::uagent_no_version(), Cookie::get(REMEMBER_ME_COOKIE_NAME)]]);

The result is nothing. Moreover when I tried to get the exact sql query the query() method gets, Instead of

       SELECT * FROM user_sessions WHERE user_name=? AND session=?

query , the

       Show Columns 

query runs. The mechanism I want implement is to get the first record in a result set for a particular query. However, as a Select query gets replaced by Show Columns query, the plan does not work as expected.

I tried to check what sql query the _read() method builds. The select query is built as expected. But when the query() method is called from within the _read() method, instead of SELECT query, Show Columns from user_sessions query runs.

I want to know whey select query is replaced by show columns query