php中如何在json中带入php变量

$st="xxxxxxx";
$st1 = json_encode($st);$bodyContent = '{
"cid":"wewewewew",
"list":[{
"siteKey":"'.$st1.'",//(这一行事例，已经证实，写法不对。不知道问题是出在拼接上，还是出在JSON中PHP的变量写法上，运行报错为，SITEKEY为空值)
"siteName":"xxxxxxx",
"transactionTime": "xxxxxxx",
"transactionNumber": "xxxxxxx",
"salesAmount": "xxxxxxx",
"totalUnits": "xxxxxxx"
}]
}';