What is wrong with this PHP code?

i tried to follow the instructions this thread: https://www.ads-software.com/support/topic/how-do-i-change-width-of-twenty-ten-to-800-pixels?replies=7

it was to change the size of the width of twentyten theme. i used the php given which was this:

<?php

add_action( 'after_setup_theme', 'child_theme_setup' );

if ( !function_exists( 'child_theme_setup' ) ):
function child_theme_setup() {
	define( 'HEADER_IMAGE_WIDTH', apply_filters( 'twentyten_header_image_width', 800 ) );
}

and i get this error:

Parse error: syntax error, unexpected 'define' (T_STRING), expecting '{' in C:\xampp\htdocs\wordpress\wp-content\themes\twentyten-child\functions.php on line 17

this is what my functions.php looks like overall (it’s a child theme):

<?php

add_filter('twentyten_header_image_height','my_header_height');
add_filter('twentyten_header_image_width','my_header_width');
function my_header_height($size){
   return 180;
}
function my_header_width($size){
   return 1920;
}

add_action( 'after_setup_theme', 'child_theme_setup' );

if ( !function_exists( 'child_theme_setup' ) ) {
function child_theme_setup()
	define( 'HEADER_IMAGE_WIDTH', apply_filters( 'twentyten_header_image_width', 822 ) );
}

there was another guy in the thread that said he also had an error and tried to give instructions on how to fix it, but it didnt work for me. can anyone help me with this?