Did you initialize a session by calling session_start() before calling session_decode() ?

Yes, session_start() is called in another page. I store the session data string in a local database and pass the session ID from page to page. 

I should note that I've used these scripts with no problems for MONTHS, I began experiencing crashes yesterday, for no reason I can identify, and it's been all downhill from there.

But does that page that calls session_decode call session_start itself?

I closed the other report since it really is same issue.
Please add a short simple script which clearly shows the
problem you're having. And all in one single script, thank you.

To answer iliaa's question: No, session_start is called on a separate page. 

This is really weird, I made up the following script as per Sniper's request:

<?
session_start();
session_register('testvar');
$testvar = "test";
$data = session_encode();
$testvar = "false";
session_decode($data);
echo("$testvar");
?>

Strangely, THIS script works fine (if it was 'broken', it should output "false", while it in fact prints "test"). 

However, I've made another script where the error is replicated:
www.dfstudios.com/session_test.php

Here's the source:

session_test.php------
<?
session_start();
session_register('testvar');
$testvar = "test";

$data = session_encode();
echo("testvar is \"$testvar\", and has been encoded: $data <br><br>");
?>
<a href="session_test2.php?<? echo("$data"); ?>>Click here for page two</a>

session_test2.php------
<?
session_decode($QUERY_STRING);
echo("testvar is \"$testvar\", and the string used for decoding is $QUERY_STRING <br><br>");
?>

Notice how $testvar is properly set on the first page and encoded properly, but in the second page $testvar is null. It seems that if session_start and session_decode are called in separate pages the decode function fails.

Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

Unless you do session_start() before doing session_decode(), the latter will fail to produce valid data. You MUST call session start before calling session_decode.