ReturnRelatedSource
Initializes `String::$_source` using the best available random number generator.
When available, `/dev/urandom` and COM gets used on *nix and [Windows systems](http://msdn.microsoft.com/en-us/library/aa388182%28VS.85%29.aspx?ppud=4), respectively. If all else fails, a Mersenne Twister gets used. (Strictly speaking, this fallback is inadequate, but good enough.)

Returns

closure Returns a closure containing a random number generator.

Source

						protected static function _source() {
		switch (true) {
			case isset(static::$_source):
				return static::$_source;
			case is_readable('/dev/urandom') && $fp = fopen('/dev/urandom', 'rb'):
				return static::$_source = function($bytes) use (&$fp) {
					return fread($fp, $bytes);
				};
			case class_exists('COM', false):
				try {
					$com = new COM('CAPICOM.Utilities.1');
					return static::$_source = function($bytes) use ($com) {
						return base64_decode($com->GetRandom($bytes, 0));
					};
				} catch (Exception $e) {
				}
			default:
				return static::$_source = function($bytes) {
					$rand = '';

					for ($i = 0; $i < $bytes; $i++) {
						$rand .= chr(mt_rand(0, 255));
					}
					return $rand;
				};
		}
	}