php提示:Call to undefined function curl_init

$ch = curl_init(); //初始化curl
curl_setopt($ch, CURLOPT_URL, $url);//设置链接
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);//设置是否返回信息
curl_setopt($ch, CURLOPT_HTTPHEADER, $header);//设置HTTP头
curl_setopt($ch, CURLOPT_POST, 1);//设置为POST方式
curl_setopt($ch, CURLOPT_POSTFIELDS, $xml_data);//POST数据
$response = curl_exec($ch);//接收返回信息
if(curl_errno($ch)){//出错则显示错误信息
 print curl_error($ch);
}
curl_close($ch); //关闭curl链接
echo $response;//显示返回信息