How do I edit a certain part of a file through php?

Question

I want to edit certain parts of a file before it gets downloaded. I want to do it via php, but i can live with javascript or flash. How wold i do it?

this is how i tried it, but it replaces the whole line and also doesnt revert back to the original

<?php
$file = 'folder/file.ext';

if($_POST['filename'] != null)
{
$exclude = "text"; 
 $lines = file($file); 
 $out = $_POST['filename']; 
 foreach ($lines as $line) { 
  if (strstr($line, $exclude) == "") { 
   $out .= $line; 
  } 
 } 
 $f = fopen($file, "w"); 
 fwrite($f, $out); 
 fclose($f); 
if (file_exists($file)) {
    header('Content-Description: File Transfer');
    header('Content-Type: application/octet-stream');
    header('Content-Disposition: attachment; filename="text.php"');
    header('Expires: 0');
    header('Cache-Control: must-revalidate');
    header('Pragma: public');
    header('Content-Length: ' . filesize($file));
    readfile($file);
    exit;
    $exclude = $_POST['filename']; 
    $lines = file($file); 
    $out = "text"; 
    foreach ($lines as $line) { 
        if (strstr($line, $exclude) == "") { 
        $out .= $line; 
    } 
    } 
 $f = fopen($file, "w"); 
 fwrite($f, $out); 
 fclose($f); 
}

}
?>

Answer 1

I think you have to download file and then modify it and save.

To read particular part of the file use fgets($file, position) If you want to place content in particular position try fseek() If you want just append file_put_content($file, $content, FILE_APPEND);

Answer 2

For this you want to learn about a few things. The first being mod_rewrite, which allows "URL Rewriting". What you can do with this, is you can make it so that when a user tried to access "http://example.com/files/file-123.txt", instead they get "http://example.com/download.php?file=file-123.txt". They won't know that they're hitting the PHP Script, but they do.

Using this, on download.php, you can do something like

$file = $_GET['file'];
if (file_exists($file)) 
{
    //Put your headers here
    $contents = file_get_contents($pathToFile);

    //Do your replacements in $contents here, but don't write to the file
    echo $contents;
}

And that way you're only modifying the file that gets sent to the user, not the file that's on your server itself.

Keep in mind that this doesn't outlay any security issues that you'll need to fix. There are a lot of them when you're doing file access, such as making sure that $_GET['file'] is a file that they're actually allowed to access, and not say "/etc/sudoers".

But I think this is the approach that you'd like to take, yes?