<?php

$file = 'example.pdf';

// get the current metadata
$command = sprintf('pdftk %s dump_data', escapeshellarg($file));
$output = array(); $return = null; exec($command, $output, $return);

//print_r($output);

if ($return) {
    throw new Exception('There was an error reading metadata from the PDF file');
}

// set any metadata values to null
foreach ($output as $index => $line) {
    if (strpos($line, 'InfoValue:') === 0) {
        $output[$index] = 'InfoValue:';
    }
}

// write the updated metadata to a file
$metadataFile = tempnam(sys_get_temp_dir(), 'pdf-meta-');
file_put_contents($metadataFile, implode("\n", $output));

// create a new PDF using the updated metadata
$tmpFile = tempnam(sys_get_temp_dir(), 'pdf-tmp-');
$command = sprintf('pdftk %s update_info %s output %s',
    escapeshellarg($file), escapeshellarg($metadataFile), escapeshellarg($tmpFile));
$output = array(); $return = null; exec($command, $output, $return);

if ($return) {
    throw new Exception('There was an error writing metadata to the PDF file');
}

// clean up the temporary files
rename($tmpFile, $file);
unlink($metadataFile);