Retrieves width and height attributes using given width and height values.

Both attributes are required in the sense that both parameters must have a
value, but are optional in that if you set them to false or null, then they
will not be added to the returned string.

You can set the value using a string, but it will only take numeric values.
If you wish to put ‘px’ after the numbers, then it will be stripped out of
the return.

function image_hwstring( $width, $height ) {
	$out = '';
	if ( $width ) {
		$out .= 'width="' . (int) $width . '" ';
	}
	if ( $height ) {
		$out .= 'height="' . (int) $height . '" ';
	}
	return $out;
}