Image Magick is a program that you can call using the system() function. You do NOT need GD installed for this to work (why does everyone thing you do??).
Your PHP script would compile a command, say like the following:
// location of actaul binary (varies depending on your system)
$convert = "/usr/X11R6/bin/convert";
$in_img = "/var/www/images/test/myimage.jpg";
$out_img = "/var/www/images/test/thumb.jpg";
$command = "$convert $in_img -geometry 200x200 $out_img";
system( $command );
What does this do? Well, it helps to read the Image Magick manual here: http://imagemagick.org/
That will help you get started with syntax and command building.