PHP - Using $this when not in object context

Question

When I try to execute this code from inherited class I got this error Using $this when not in object context here my code

abstract class Connection {

    private static $host;
    private static $username;
    private static $password;
    private static $database;

    public function __construct() {
        self::$host = 'localhost'; // database server address
        self::$username = 'root'; //database server username;
        self::$password = ''; //database server password;
        self::$database = 'oms'; //database name
    }

    private function connect() {
        return new PDO('mysql:host=' . Connection::$host . ';dbname=' . Connection::$database . '', Connection::$username, Connection::$password);
    }

    protected function execute($sql) {
        $this->connect();
        return $this->connect()->query($sql);
    }

}

what are the reason for that? i don't use any static method in Connection class. SoWhy do give this error?

Answer 1

This is probably what you want:

abstract class Connection {

    protected static $host;
    protected static $username;
    protected static $password;
    protected static $database;
    protected static $connection;

    public static function load() {
        self::$host = 'localhost'; // database server address
        self::$username = 'root'; //database server username;
        self::$password = ''; //database server password;
        self::$database = 'oms'; //database name
    }

    protected static function connect() {
        // Only create the connection once
        if (!self::$connection) {
            self::$connection = new PDO('mysql:host=' . self::$host . ';dbname=' . self::$database . '', self::$username, self::$password);
        }
        return self::$connection;
    }

    public static function execute($sql) {
        return self::$connect()->query($sql);
    }

}

// Because we can't use an abstract class
class ConcreteConnection extends Connection {}

// Execute a SQL call
ConcreteConnection::execute("SELECT * FROM `table`");

But then what you'll have is a Singleton Pattern which is a pain for testing and changing later. I'd recommend you make it like this:

abstract class Connection {

    protected $host;
    protected $username;
    protected $password;
    protected $database;
    protected $connection;

    public function _construct($host, $username, $password, $database) {
        $this->host = $host;
        $this->username = $username;
        $this->password = $password;
        $this->database = $database;
    }

    protected function connect() {
        if (!$this->connection) {
            $this->connection = new PDO('mysql:host=' . $this->host . ';dbname=' . $this->database . '', $this->username, $this->password);
        }
        return $this->connection;
    }

    public function execute($sql) {
        return $this->connect()->query($sql);
    }

}

// Because we can't use an abstract class
class ConcreteConnection extends Connection {}

// Inject our parameters into our class
$connection = new ConcreteConnection('host', 'username', 'password', 'database');
// Execute a SQL call
$connection->execute("SELECT * FROM `table`");