This is a very useful function in PHP, I found it on php.net
PHP provides function to get the width and height of an image. The getimagesize() function will determine the size of image file including flash file(swf).

list($width, $height, $type, $attr) = getimagesize("image_name.jpg");

This is a sample code, change image_name to your image name and test it!

<?php

list($width, $height, $type, $attr) = getimagesize("image_name.jpg");

echo "Image width " .$width;
echo "<BR>";
echo "Image height " .$height;
echo "<BR>";
echo "Image type " .$type;
echo "<BR>";
echo "Attribute " .$attr;

?>


When you run this script you will see the result like this

Image width 379
Image height 344
Image type 2
Image attribute width="379" height="344"

You will get the width, height, type of an image and also attribute of an image, I use this function in my image upload form.
Type of an image you can see from table below
You can read a full manual here