How do I convert a PDF document to a preview image in PHP?

You need ImageMagick and GhostScript

<?php
$im = new imagick('file.pdf[0]');
$im->setImageFormat('jpg');
header('Content-Type: image/jpeg');
echo $im;
?>

The [0] means page 1.

For those who don't have ImageMagick for whatever reason, GD functions will also work, in conjunction with GhostScript. Run the ghostscript command with exec() to convert a PDF to JPG, and manipulate the resulting file with imagecreatefromjpeg().

Run the ghostscript command:

exec('gs -dSAFER -dBATCH -sDEVICE=jpeg -dTextAlphaBits=4 -dGraphicsAlphaBits=4 -r300 -sOutputFile=whatever.jpg input.pdf')

To manipulate, create a new placeholder image, $newimage = imagecreatetruecolor(...), and bring in the current image. $image = imagecreatefromjpeg('whatever.jpg'), and then you can use imagecopyresampled() to change the size, or any number of other built-in, non-imagemagick commands

You can also get the page count using

$im->getNumberImages();

Then you can can create thumbs of all the pages using a loop, eg.

'file.pdf['.$x.']'