Hi I need help running multiple commands when using PHP

using the exec function in PHP

these are the commands that I need to execute
I need PHP to run the following commands
This will encode the video as FLV
$output = '0000005'; // A number between 0000000 and 9999999
$input= 'donewithform.flv'; // Will be chosen with a form
--

ffmpeg -i $input -ar 22050 -ab 32 -f flv -s 320x240 $output.flv

--
This will Take a screenshot
--

ffmpeg -i $output.flv -an -ss 00:00:45 -an -r 1 -vframes 1 -y -s 150x100 $output.jpg

--
so how can this be done I am confused can someone help me

also ffmpeg is at /usr/bin/ffmpeg/
output needs to be saved at /home/forbushj/public_html/videos/
input needs to be uploaded to /home/forbushj/convert/ and will be converted and outputed to /home/forbushj/public_html/videos/
can someone help me?

I've found out a little bit since the first post I have made some code but I still need some help
%d I think is what makes the filename the next number up if not please help me or if not please help me
I need this to do one command after the other but I need it to make sure it is ready before starting the next command

PHP Code:

<?php 
$input = $_GET['input'];
$output = '%d'; //hopefully that will make the file name a number
exec 'ffmpeg -i /home/forbushj/public_html/uploadvidd/'.$input.' -ar 22050 -ab 32 -f flv -s 320x240 /home/forbushj/public_html/2vidd/'.$output.'.flv';
exec 'ffmpeg -i /home/forbushj/public_html/uploadvidd/'.$input.' -an -ss 00:00:45 -an -r 1 -vframes 1 -y -s 150x100 /home/forbushj/public_html/2vidd/'.$output.'.jpg';
?>