php中的pdftk无法正常工作

here is the code.... it is not able to merge two pdf files ...can anyone find what the problem is in the code... thanks for ur time....

You're using the wrong syntax.

Right code should looks like such this:

// prepare command escaping paths of your files
$parts = '';
foreach( array( 'file1.pdf', 'file2.pdf' ) as $part )
    $parts .= escapeshellarg( $part ) . ' ';

// build command
$command = sprintf( 'pdftk %s cat output %s', $parts, escapeshellarg($output) );

// launch it
system( $command, $returnvalue );
if( $returnvalue )  printf 'There was an error.';

Next time note: please choose a more-specific title and remove unnecessary code.