The following function applies a callback to each image in an Imagick object.

<?php
function imagickMap($callback, $im)
{
    $im->setFirstIterator();

    do
    {
        $callback($im);
    }
    while ( $im->nextImage() );
}
?>

E.g. to convert a series of images to grayscale:

<?php
imagickMap(
    function($im)
    {
        $im->setImageType(Imagick::IMGTYPE_GRAYSCALEMATTE);
    },
    $im
);
?>

Convert PDF to JPG page-wise:

<?php
$i = 0;

$imagick = new Imagick();
$imagick->readImage('myfile.pdf');
while($imagick->hasNextImage()) {
    $imagick->writeImage(++$i.'-converted.jpg', false);
    $imagick->nextImage();
}
?>

See also: http://php.net/manual/en/imagick.readimage.php