code: public static function init()
  	{
  		// if initializated, just return
  		if (self::$utmCookie !== null) {
  			return;
  		}
  		
  		self::initStaticValues();
  
  		// utm from _COOKIE
  		$utmCookieFilter = filter_input(INPUT_COOKIE, self::$utmCookieName, FILTER_SANITIZE_STRING, FILTER_REQUIRE_ARRAY);
  		if (false === is_array($utmCookieFilter)) {
  			$utmCookieFilter = [];
  		}
  		$utmCookie = self::removeNullValues($utmCookieFilter);
  
  		// utm from _GET
  		$utmGetFilter = filter_input_array(
  				INPUT_GET, 
  				[
  					'utm_campaign' => FILTER_SANITIZE_STRING, 
  					'utm_medium'   => FILTER_SANITIZE_STRING, 
  					'utm_source'   => FILTER_SANITIZE_STRING, 
  					'utm_term'     => FILTER_SANITIZE_STRING, 
  					'utm_content'  => FILTER_SANITIZE_STRING
  				]
  		);
  		if (false === is_array($utmGetFilter)) {
  			$utmGetFilter = [];
  		}
  		$utmGet = self::removeNullValues($utmGetFilter);
  		
  		if (count($utmGet) !== 0 && self::$overwrite === true) {
  			$utmCookieSave = array_merge(self::$utmCookie, $utmGet);
  		} else {
  			$utmCookieSave = array_merge(self::$utmCookie, $utmCookie, $utmGet);
  		}
  
  		if (count($utmGet) !== 0) {
  			self::save($utmCookieSave);
  		} else {
  			self::$utmCookie = $utmCookieSave;
  		}
  	}
docs: Initialize. Get values from _GET and _COOKIES and save to UtmCookie. Init self::$utmCookie value.
  
  @return void