imagetruecolortopalette() is:
void imagetruecolortopalette ( resource image, bool dither, int ncolors)

but the function seems to ignore my ncolors input. ncolors  should be the maximum number of colors in the output image. (and I cannot tell if dither works)

Test script:
<?php
header( "Content-type: image/png" );

$medimg = $_SERVER['DOCUMENT_ROOT'] . "/riverdata/images/world_map_med.gd2";
$img = ImageCreateFromGD2Part( $medimg, 900, 500, 800, 600 );

//Avoid bug #19418
$medimg = ImageCreateTrueColor( 800, 600 );
ImageCopy( $medimg, $img, 0, 0, 0, 0, 800, 600 );
ImageDestroy( $img );
$img = $medimg;

ImageTrueColorToPalette( $img, FALSE, 2 );

ImagePNG( $img );
ImageDestroy( $img );
?>

You can find the GD2 image that I am using here:
http://144.92.10.251/riverdata/images/world_map_med.gd2

The previous episode in this saga can be seen here:
http://bugs.php.net/bug.php?id=19704