PHP ERROR : Call to undefined function curl_init()

/**
* @Author:      HTL
* @Email:       Huangyuan413026@163.com
* @DateTime:    2016-12-28 11:36:03
* @Description: 获取远程内容
*/
function _file_get_contents_curl($url) {
$ch = curl_init();
curl_setopt($ch, CURLOPT_AUTOREFERER, TRUE);
curl_setopt($ch, CURLOPT_HEADER, 0);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, TRUE);
$data = curl_exec($ch);
curl_close($ch);
return $data;
}

print_r(_file_get_contents_curl("http://baidu.com"));