PDO based Database Wrapper class

DB.php

<?php

class Db
{
    private $conn = null;
    private $dsn;
    private $user;
    private $password;

    public function __construct($dsn, $user, $password)
    {
        $this->dsn = $dsn;
        $this->user = $user;
        $this->password = $password;
    }
    
    private function connect()
    {
        if(!isset($this->conn)){
            try{
                $this->conn = new PDO($this->dsn, $this->user, $this->password);
                $this->conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
                $this->conn->setAttribute(PDO::ATTR_EMULATE_PREPARES, FALSE);
            }catch(PDOException $e){
                $this->disconnect();
                echo $e->getMessage();
            }
        }
        return $this->conn;
    }
    
    public function disconnect(){
        $this->conn = null;
    }
    
    public function execute($sql, $params = null)
    {
        try{
            $conn = $this->connect();
            $stmt = $conn->prepare($sql);
            $stmt->execute($params);
        }catch(PDOException $e){
            $this->disconnect();
            echo $e->getMessage();
        }
    }
    
    public function all($sql, $params = null, $fetch_style = PDO::FETCH_OBJ)
    {
        $result = null;
        try{
            $conn = $this->connect();
            $stmt = $conn->prepare($sql);
            $stmt->execute($params);
            $result = $stmt->fetchAll($fetch_style);
        }catch(PDOException $e){
            $this->disconnect();
            echo $e->getMessage();
        }
        return $result;
    }
    
    public function row($sql, $params = null, $fetch_style = PDO::FETCH_OBJ)
    {
        $result = null;
        try{
            $conn = $this->connect($conn_name);
            $stmt = $conn->prepare($sql);
            $stmt->execute($params);
            $result = $stmt->fetch($fetch_style);
        }catch(PDOException $e){
            $this->disconnect();
            echo $e->getMessage();
        }
        return $result;
    }
    
    public function one($sql, $params = null)
    {
        $result = null;
        try{
            $conn = $this->connect();
            $stmt = $conn->prepare($sql);
            $stmt->execute($params);
            $result = $stmt->fetch(PDO::FETCH_NUM);
            $result = $result[0];
        }catch(PDOException $e){
            $this->disconnect();
            echo $e->getMessage();
        }
        return $result;
    }
}