下記の方法はどうでしょうか?
function test($url){
$headers = array("幅: bytes=0-32768");
$curl = curl_init($url);
curl_setopt($curl, CURLOPT_HTTPHEADER, $headers);
curl_setopt($curl, CURLOPT_RETURNTRANSFER, 1);
$data = curl_exec($curl);
curl_close($curl);
return $data;
}
$start = microtime(true);
$url = "http://画像の置いてあるURL";
$raw = test($url);
$im = imagecreatefromstring($raw);
$width = imagesx($im);
$height = imagesy($im);
$time = round(microtime(true) - $start, 5);
echo $width." x ".$height." ({$time}s)";
出力例
640 x 480 (1.09006s)