没有从Ajax调用获取JSON数组

您误解了Javascript和php的关系；该php仅在页面加载时呈现一次，因此您无法echo在Javascript success处理程序中返回php数据。

相反，您的php脚本输出的所有内容都将在Javascript变量中可用：

success: function(vnvlist) {
           //     ^^^^^^^ this is what has been outputted by php, the
           //             json already parsed

           // your data is available in `vnvlist`
           // var prevnvlist = '';

           // with dataType='json' you don't need to parse anything
           // as jQuery will parse it for you
           // var vnvlist = JSON.parse(prevnvlist);

           // so all you need is this...
           alert('success');
           for (var x = 1; x <= vnvlist.length; x++) {
             var vnv = vnvlist[x]['VnVMethod'];
             vnvSel.options[vnvSel.options.length] = new Option(vnv, vnv);
           }
         },

并且您需要确保php仅输出您的json字符串：

...
// The following line when read in the log file shows a valid JSON array
$log->lwrite('array '.json_encode($dataVnvMethods));

// output your data so that it is available on the client-side
echo json_encode($dataVnvMethods);

$log->lwrite('Ending debug');
...