abstract class Expression {
    private static $keycount=0;
    private $key;
    abstract function interpret( Context $context );

    function getKey() {
        if ( ! isset( $this->key ) ) {
            self::$keycount++;
            $this->key=self::$keycount;
        }
        return $this->key;
    }
}

class LiteralExpression extends Expression {
    private $value;

    function __construct( $value ) {
        $this->value = $value;
    }

    function interpret( Context $context ) {
        $context->replace($this, $this->value );
    }
}

$context = new Context();
$literal = new LiteralExpression( 'four');
$literal->interpret( $context );