Questions and postings pertaining to the usage of ImageMagick regardless of the interface. This includes the command-line utilities, as well as the C and C++ APIs. Usage questions are like "How do I use ImageMagick to create drop shadows?".
You are using Imagemagick with the command line not Imagick.
Your code seems a bit over complicated as you do not need all the " and .
Try writing it like this as you can echo the command and see it contains what you expect, I also create my names outside of the exec() as I think it is easier:
// Untested
// Paths can be relative which will clean up these imagempaths
$input = '/home/isocia5/public_html/fbcalendar/images/exports/cal_print_bot_1.jpg';
$output = "/home/isocia5/public_html/fbcalendar/src/php/upload/temp/blank_image_'.$canvas_id.'.jpg";
// I assume you a placing one image onto a plain background - if it is I would use -extent as it is a lot simpler
$cmd =" -size {$final_image_w}x{$final_image_h} xc:white $input -geometry +1659+500 -composite ";
echo $cmd.'<br>';
exec("convert $cmd $output ",$out,$err);