致命错误：调用未定义的函数 curl_init（）

<?php
$filename = "xx.gif";
$handle = fopen($filename, "r");
$data = fread($handle, filesize($filename));

// $data is file data
$pvars   = array('image' => base64_encode($data), 'key' => IMGUR_API_KEY);
$timeout = 30;
$curl    = curl_init();

curl_setopt($curl, CURLOPT_URL, 'http://api.imgur.com/2/upload.xml');
curl_setopt($curl, CURLOPT_TIMEOUT, $timeout);
curl_setopt($curl, CURLOPT_POST, 1);
curl_setopt($curl, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($curl, CURLOPT_POSTFIELDS, $pvars);

$xml = curl_exec($curl);

curl_close ($curl);

var_dump($xml);
?>

我正在玩Imgur API，但它似乎不起作用。PHP.net 说这是PHP5，但我的主人说它不是。我怎样才能做到这一点？curl_init()