PHP - Using $this when not in object context - oop

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`");