PHP exec（）命令无法打开文件

Why can't one of my PHP exec() commands open a file?

In my PHP script, there are three exec() commands, all of which can open files located in $_SERVER['DOCUMENT_ROOT']. Two of the commands can also open files in the sub-directories (all directories and files have the same permissions), but one of the commands (case "3") cannot. I tested the 3 commands in terminal as the same Apache user, in the same sub-directories, and they all worked. I also tested the script in the PHP CLI and they worked there as well. Here is a snippet of the code:

switch($_POST['Convert']) 
    {
        case "1":
            $FileNameConvert .= ".pdf";
            $command = "convert '{$FileName}' '{$FileNameConvert}'";
            break;
        case "2":
            $FileNameConvert .= ".html";
            $command = "pdftohtml '{$FileName}' '{$FileNameConvert}'";
            break;
        case "3":
            $FileNameConvert = "RotatedPDF.pdf";
            $command = "pdftk '{$FileName}' cat 1-endsouth output '{$FileNameConvert}'";
            break;
        default:
            echo "error";
            exit();
    }
    echo exec($command,$op,$result);

Apache's error log shows:

I/O Error: Couldn't open file '$FileName': No such file or directory.
Error: Unable to find file.
Error: Failed to open PDF file: 
$FileName  

Why does the following command not work?

$command = "pdftk '{$FileName}' cat 1-endsouth output '{$FileNameConvert}'";