Just a warning: don't use Eero Niemi's code (identifyImage with pingImage) if you just want to get the image width and height, because it will actually be slower than reading the whole image into memory - about 10x slower!

Correct code should be:

<?php

  $image = new Imagick();
  $image->pingImage($file);
  $width = $image->getImageWidth();
  $height = $image->getImageHeight();

?>

(this is around 15 times faster than reading the image in memory)

If you don't sure whether the file exists or not or maybe it's broken, then you should use try - catch construction. It prevents code fails, when code stops execution after call of pingImage (if the file doesn't exist or it's broken).

<?php
$im = new Imagick();
try {
    $im->pingImage('3.jpg');
}
catch(ImagickException $e) {
    echo "image doesn't exist";
}
?>