Imagepng gives no output and no error message if the /tmp folder is full.
The header is writed, but there is no output at all.
Would be nice if there is a message if the file can't be created.

$im = ImageCreate($xx, $yy) or die ("Cannot Initialize new GD image stream");
$black = ImageColorAllocate( $im, 0, 0, 0 );
$white = ImageColorAllocate( $im, 255, 255, 255 );
ImageLine( $im, 0, 0, $xx-1, $yy-1, $white);
header("Content-Type: image/png");
ImagePng( $im );