class MyConstruct {
	public $param1;
	public $param2;
	public $param3;
 function __construct($text1_param1,$text2_param2, $text3_param3){
	$this->param1 = $text1_param1;
	$this->param2 = $text2_param2;
	$this->param3 =$text3_param3;	
             }
 function pr_result() {
 echo $this->param1."<br>";
 echo $this->param2."<br>";
 echo $this->param3;
}
	
}
$la = new MyConstruct('{"first":"name","second":"surname","thi  rd":{"sub1":1,"sub2":2,"sub3":3}}'); 
 $la->pr_result();

И что нужно сделать чтобы получилось так : 
$la->get('first'); // Вернёт 'name'
$la->get('third')->get('sub2'); // Вернёт 2

$la->get('first'); // Вернёт 'name'

$la->get('third')->get('sub2'); // Вернёт 2

class MyConstruct {
	public $param1;
	public $param2;
	public $param3;

 function __construct($text1_param1){
	
	$this->param1 =json_decode( $text1_param1);
	}
 function get() { 
   echo $this->param1->second."<BR>"; // выводит name
   echo $this->param1->first;                // выводит surname
 }
	
}
$la = new MyConstruct('{"first":"name","second":"surname","third":{" ;sub1":1,"sub2":2,"sub3":3}}'); 
$la->get();