getimagesize.php

<?php
$size = getimagesize('http://free-photos-ls01.gatag.net/images/lgf01a201304271400.jpg');
var_export($size);

/* output
array (
  0 => 1024,
  1 => 768,
  2 => 2,
  3 => 'width="1024" height="768"',
  'bits' => 8,
  'channels' => 3,
  'mime' => 'image/jpeg',
)
*/