php中出现：Call to undefined function curl_init()的解决方法

$ch = curl_init();
curl_setopt ($ch, CURLOPT_URL, "http://www.zend.com/");
curl_setopt ($ch, CURLOPT_HEADER, 0);
curl_exec ($ch);
curl_close ($ch);
?>

参见：curl_close(), curl_setopt()