To find which variables are not serialized correctly, I created a block with this code, using the PHP input format:

$result = db_query('SELECT * FROM {variable}');
    while ($variable = db_fetch_object($result)) {
    
      $variables[$variable->name] = @unserialize($variable->value);
     
      if ($variables[$variable->name] === FALSE) {
       print "<hr/>Unserialize Error for variable:". $variable->name . '='. $variables[$variable->name] . "<br/>". $variable->value;
      }
    }

It's much more practical (and safe) than messing up with Drupal core files.

I don't like the above methods of debugging because you don't want to suppress the unserialize error output.

I like to use this bit of PHP:

#!/usr/bin/php

<?php
$conn = new MySQLi('localhost', 'root', '', 'drupal_gypsy');

if (!$conn) {
  die("MySQL error: #{$conn->connect_errno}: {$conn->connect_error}");
}

$result = $conn->query("select * from gypsy_variable");

while ($row = $result->fetch_row()) {
  print $row[0];
  $data = unserialize($row[1]);
  print "    ";
  print $data . "\n";
  print "-------------------\n";
}

It shows I have one corrupted row whose data is this:

a:27:{s:17:"zen_block_editing";i:0;s:14:"zen_breadcrumb";s:2:"no";s:24:"zen_breadcrumb_separator";s:5:" › ";s:19:"zen_breadcrumb_home";i:1;s:23:"zen_breadcrumb_trailing";i:1;s:20:"zen_breadcrumb_title";i:0;s:20:"zen_rebuild_registry";i:0;s:14:"zen_wireframes";i:0;s:11:"toggle_logo";i:1;s:11:"toggle_name";i:1;s:13:"toggle_slogan";i:0;s:14:"toggle_mission";i:0;s:24:"toggle_node_user_picture";i:0;s:27:"toggle_comment_user_picture";i:0;s:13:"toggle_search";i:0;s:14:"toggle_favicon";i:1;s:20:"toggle_primary_links";i:1;s:22:"toggle_secondary_links";i:0;s:12:"default_logo";i:0;s:9:"logo_path";s:43:"sites/gypsyfalls/files/hg_template_logo.png";s:11:"logo_upload";s:0:"";s:15:"default_favicon";i:0;s:12:"favicon_path";s:41:"sites/gypsyfalls/themes/gypsy/favicon.ico";s:14:"favicon_upload";s:0:"";s:14:"default_banner";i:0;s:11:"banner_path";s:0:"";s:13:"banner_upload";s:0:"";}

Good lord! Where's the problem? Well, the error from unserialize says:

Notice: unserialize(): Error at offset 108 of 876 bytes in /data/www/gypsyfalls/
find_bad_var.php on line 14

Using this and "cut" under Linux, I can at least get the context:

$ cut --bytes=104-112 bs2
 › ";s:

Text search for that and I see it's this guy:

s:5:" › "

That's an s:3 not an s:5!!

VERY DISTURBING that there's a PHP or Drupal bug that's corrupting my variables table! Anybody know of an existing ticket somewhere I can pile onto?

Thanks Johnbarclay, xtfer, larowlan and tobiasb
This code is very much helpful for me. I have added extra piece of info please see section
How you can remove the above error

Regards,
Deb

Do not alter values manually
You can use Online Sterilizer test serialize online

Thanks.

Thanks all, I was able to identify and fix a unserialize() error I was faced with.

I used the code posted by @gbell12 by putting it into a self-contained php file in my docroot. Also, make sure you are selecting from the variables table from your drupal database.

In the code, make sure to enter your password here:

$conn = new MySQLi('localhost', 'your_user', 'your_password', 'your_drupal_database');

Then, view the source of the output from your new php file, and you will see the serialize error right underneath the data row that has the data that needs to be fixed. In my case, it was an email address that I had replaced by manually editing the sql files (not a great idea unless you have to), and the new email address was shorter than the original one, so the s: value was wrong - therefore the error.

Thanks a lot. johnbarclay!

// Added by Deb
-- Open includes/bootstrap.inc file drupal 6.22
-- Go to line no 568
-- Paste below code after the line no 568 or "$variables[$variable->name] = unserialize($variable->value);" line.
if ($variables[$variable->name] === FALSE) {
variable_set($variable->name, ""); //here all the blank variable will be true
}
-- After changing all the necessary variable in database (variable table), please delete the edited code and clear the cache and truncate the watchdog table and refresh the page again.
--Check in admin "recent log entries"

Thank you everyone for your solutions and clear explanations in this post. Using what I learned here I came up with a quick and painless fix that worked for me.

Setup: I had to migrate my site to a new server, and when I did I got many serialized variable errors. Here is how I resolved the problem (D6):

(PLEASE NOTE: I highly recommend backing up your database before making any manual corrections to entries, or at least copy the contents of the problem entries to a text editor for safe keeping until you know you've done a good thing).

Step 1: Download and instal the Variable Check module (http://drupal.org/project/variablecheck)

Step 2: Check the module's report page to see what is returning errors (admin/reports/variablecheck)

Step 3: Login to PHPmyAdmin and do a database wide search for the name of the variable returning the error and choose to edit it

Step 4: Copy the value of the field.

Step 5: Open the "Serialize and Unserialize web tool" in a separate window (http://unserialize.net/serialize)

Step 6: Select to translate from "PHP Serialize" to "PHP Serialize"

Step 7: Paste the bad entry in the first text box and click the "Process" button and Voila! your serialized entry has been repaired!

Step 8: Copy the new entry back into your database and save the record. Lather. Rinse. Repeat.

https://drupal.org/project/variablecheck

This worked for me on a D6 site. Identified variables, then used Devel variable editor to delete those that were unserialized.

Seem to have come from a module that was removed improperly and left remnants in the DB.

I got 3 variable table "unserialize" errors from bootstrap.inc after migrating a production site down to my Mac. I wanted to get rid of these notices on my local dev site so they wouldn't distract me from other messages during module development. I used the Variable Check module as has been suggested here, and it revealed three variable rows with issues:

maintenance_mode_message
site_mail
webform_default_from_address

I found I could fix these without doing any direct deletes against the variables table.

Using the Admin interface, I went to the Configuration->Maintenance Mode page and saw that there was no maintenance mode message. I added a message, saved, cleared cache, and one notice went away. I did the same for the other two: site_mail was the email address in Configuration->Site Information, and webform_default_from_address was the email "from address" in Configuration->Webform Settings. In both of these cases, the value was missing and adding a new value, saving, and clearing the cache fixed the problem.

$databases = array (
  'default' =>
    array (
      'default' =>
        array (
          'driver' => 'mysql',
          'database' => 'martin',
          'username' => 'root',
          'password' => '1111',
          'host' => 'localhost',
          'port' => '',
          'prefix' => 'dp614_',
<strong>          'charset' => 'utf8mb4',
          'collation' => 'utf8mb4_general_ci',</strong>
        ),
    ),
);