If you use PHP, you may need to use cURL, which is one of the most popular extension. PHP CURL functions are available through the use of libcurl, a library created by Daniel Stenberg, and allow you to connect and communicate with web servers using many
different types of protocols.
Assume you have already setup LAMP. To install or setup cURL on your Linux machine like Ubuntu, run the following line of shell command in your terminal:
sudo apt-get install curl libcurl3 libcurl3-dev php5-curl
Now you have PHP cURL installed, the next thing you …

Preparing Your system
sudo apt-get install gs pdftk
Now we will see one example how to combine pdf files
let us assume we have 1.pdf,2.pdf,3.pdf files and now we need to cobmine this files using the following command
gs -dNOPAUSE -sDEVICE=pdfwrite -sOUTPUTFILE=combinedpdf.pdf -dBATCH 1.pdf 2.pdf 3.pdf
the output file will be combinedpdf.pdf
Rename “1.pdf 2.pdf 3.pdf” to whatever your file names are (separate your file names with a space).
If you want to know more options available for gs command check main page