function parse_2words($str){
	$new = "";
	$parse = explode(" ", $str);
	for($i=0; $i<count($parse); $i+=2){
		$new .=  $parse[$i]." ".$parse[$i+1].", ";
	}
	return trim($new, ", ");
}

$etiket = "Ajay Devgn Vijay Salgaonkar Shriya Saran Nandini Salgaonkar Tabu Meera Deshmukh Rajat Kapoor Mahesh Deshmukh Ishita Dutta Anju Salgaonkar Prasanna Ketkar Sr. PI Shrikant Prabhu Prathamesh Parab"; 

echo parse_2words($etiket);