Below is the php PDF merge command.
$fileArray= array("name1.pdf","name2.pdf","name3.pdf","name4.pdf");
$datadir = "save_path/";
$outputName = $datadir."merged.pdf";
$cmd = "gs -q -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -sOutputFile=$outputName ";
//Add each pdf file to the end of the command
foreach($fileArray as $file) {
$cmd .= $file." ";
}
$result = shell_exec($cmd);
I forgot the link from where I found it, but it works fine.
Note: You should have gs (on linux and probably Mac), or Ghostscript (on windows) installed for this to work.