CURL POST请求出现问题,获取CURL_setopt()参数警告

尝试使用CURL发送带有XML正文的post请求,但不断出现以下错误:,

Warning: curl_setopt() expects parameter 2 to be long, string given

我使用下面的卷曲要求,

$ch = curl_init(POST_URL);

curl_setopt($ch, CURLOPT_MUTE, 1);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 1);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 1);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: text/xml'));
curl_setopt($ch, CURLOPT_POSTFIELDS, "$xml");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);

$output = curl_exec($ch);
curl_close($ch);