<?php

$str = '[{"x":"12345","y":"john"},{"x":"12345","y":"stars"}]';


function getCSV($x)
{
    $arr = json_decode($x);
    $y = array();
    foreach($arr as $obj){
        $y[] = $obj->y;
    }
    echo implode(',', $y);
}

getCSV($str);

john,stars