(PECL imagick 2.0.0)
Imagick::quantizeImage — Analyzes the colors within a reference image
$numberColors
    , int $colorspace
    , int $treedepth
    , bool $dither
    , bool $measureError
   )
numberColors
colorspace
treedepth
dither
measureError
   Returns TRUE on success.
  
Throws ImagickException on error.
Example #1 Imagick::quantizeImage()
<?php
function quantizeImage($imagePath, $numberColors, $colorSpace, $treeDepth, $dither) {
    $imagick = new \Imagick(realpath($imagePath));
    $imagick->quantizeImage($numberColors, $colorSpace, $treeDepth, $dither, false);
    $imagick->setImageFormat('png');
    header("Content-Type: image/png");
    echo $imagick->getImageBlob();
}
?>