running external program via PHP on Linux

I need an expert's help. I need to execute a program via php script. It works well on Windows, but not on Linux. The command is also OK in shell but won't run via php it can't run. Also php can run the Linux commands, but I need to run another program pdf2swf. please help me to solve this.

for ($i = 0; $i <= $_REQUEST["n_o_p"]; $i++) { 
  $oExec = exec($_SESSION['path']."pdf2swf -p ".$开发者_如何学编程i."-".$i." ".$_SESSION['path']."upload/".$_SESSION['f_n']." -o ".$_SESSION['path'].$tmp_dir."/data/pages/document".$i.".swf");
}

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<?php shell_exec('command'); ?>