Calling getimagesize() returns an array:
Array
(
[0] => 300
[1] => 200
[2] => 2
[3] => width="300" height="200"
[bits] => 8
[channels] => 3
[mime] => image/jpeg
)
The value in index 3 is a text string that expresses the image's width and height.
So:
<img src="image.jpg" alt="Descriptive text" <?php $dimensions = getimagesize("image.jpg"); echo "$dimensions[3]"; ?> />