<?php

class xpto
{
	public $id = null;

	public function __construct()
	{
	}

	public static function getMyID()
	{
		return $this->id;
	}
}

function instance($xpto = null)
{
	static $result = null;

	if (is_null($result) === true)
	{
		$result = new xpto();
	}

	if (is_object($result) === true)
	{
		$result->id = strval($xpto);
	}

	return $result;
}

echo var_dump(instance()->getMyID()) . "\n"; // null
echo var_dump(instance('dev')->getMyID()) . "\n"; // dev
echo var_dump(instance('prod')->getMyID()) . "\n"; // prod
echo var_dump(instance()->getMyID()) . "\n"; // null

?>

Fatal error: Using $this when not in object context on line 13