Q:

How do you append images that are already created with appendImage?

The php website lists the following example:
 <?php

/* Create new imagick object */
$im = new Imagick();

/* create red, green and blue images */
$im->newImage(100, 50, "red");
$im->newImage(100, 50, "green");
$im->newImage(100, 50, "blue");

/* Append the images into one */
$im->resetIterator();
$combined = $im->appendImages(true);

/* Output the image */
$combined->setImageFormat("png");
header("Content-Type: image/png");
echo $combined;
?>

How do I use an image generated from a URL instead, such as 
$image = new Imagick("sampleImage.jpg");

so that I could append the loaded images instead of using newImage()

A:

Use Imagick::addImage to "combine" the various Imagicks into one and use appendImages afterwards, for example (addapted from here):
<?php
$filelist = array("fileitem1.png","fileitem2.png","fileitem3.png");

$all = new Imagick();

foreach($filelist as $file){
    $im = new Imagick($file);       
    $all->addImage($im);
}
/* Append the images into one */
$all->resetIterator();
$combined = $all->appendImages(true);

/* Output the image */
$combined->setImageFormat("png");
header("Content-Type: image/png");
echo $combined;
?>

