PHP GD header issue

A couple of months ago I enabled GD on localhost to play around with it a bit, used various scripts found online to get a better understanding of what it does, all good and well.

But now I feel like I'm descending into insanity here because when trying to repeat the above with the exact same scripts I keep getting header errors, both on local and remote host. I figure I must have somehow completely forgotten how do do it right so I need a brief refresher on this.

For example, this was the first script I ever tried:

$my_img = imagecreate( 200, 80 );
$background = imagecolorallocate( $my_img, 0, 0, 255 );
$text_colour = imagecolorallocate( $my_img, 255, 255, 0 );
$line_colour = imagecolorallocate( $my_img, 128, 255, 0 );
imagestring( $my_img, 4, 30, 25, "thesitewizard.com",
$text_colour );
imagesetthickness ( $my_img, 5 );
imageline( $my_img, 30, 45, 165, 45, $line_colour );

header( "Content-type: image/png" );
imagepng( $my_img );
imagecolordeallocate( $line_color );
imagecolordeallocate( $text_color );
imagecolordeallocate( $background );
imagedestroy( $my_img );

Source

Worked fine back then, now when I run it I get this:

Warning: Cannot modify header information - headers already sent by (output started at H:\xampp\htdocs\tests\script.php:1) in H:\xampp\htdocs\tests\script.php on line 11 �PNG ��� IHDR�������P���!�#��� PLTE���������M�)B����IDATH��1�0EA��N�����:��8Q�JN�d�Z����! � ���k�Z⊜���k��g�˕��|K$�M��|)��$ů8DfRQB���-��[I��