If you have allow_url_fopen set to true:

$url = 'http://example.com/image.php';

$img = '/my/folder/flower.gif';

file_put_contents($img, file_get_contents($url));

Else use cURL:

$ch = curl_init('http://example.com/image.php');

$fp = fopen('/my/folder/flower.gif', 'wb');

curl_setopt($ch, CURLOPT_FILE, $fp);

curl_setopt($ch, CURLOPT_HEADER, 0);

curl_exec($ch);

curl_close($ch);fclose($fp);