Please try using this CVS snapshot:

  http://snaps.php.net/php5.2-latest.tar.gz
 
For Windows:
 
  http://snaps.php.net/win32/php5.2-win32-latest.zip

PHP Version 5.2.1RC4-dev,  php5.2-200701241730

Corrects the issue.

2 things:

1) Can we know what was the cause of the issue in case we need to do a work around?

2) When is PHP 5.2.1 scheduled to be released?

>1) Can we know what was the cause of the issue in case we
> need to do a work around?
I can't tell that having just the information you've provided.

>2) When is PHP 5.2.1 scheduled to be released?
Soon. RC3 is expected to be the last release candidate.

Update.  I am still running 5.2-latest, however, I changed the script to have numOfIterations be equal to 1000000.  The script began the old behavior of ballooning and trying to consume as much memory as possible and the system started swapping and becoming unresponsive.

With 1000000 elements, the memory requirements should still be around:

1000000 X 66 = 66000000bytes = 62megabytes

The array is copied twice, and that should be 124megs.  We're continuing to see memory consumption skyrocket and consume all memory.

This does not occur when the number of elements is 100000.

Cannot reproduce.

100000 - 25.6Mb
1000000 - 256Mb

Use memory_get_usage() to get the memory usage.

I get this output with fatal error:
Fatal error: Allowed memory size of 536870912 bytes exhausted (tried to allocate 18874411 bytes) in Unknown on line 0

With this script:
<?php

session_start();
$myarray = array();

$numOfIterations = 1000000;
for($counter = 0; $counter < $numOfIterations; $counter++) {
  $myarray[$counter] = 'jkdfjklsdfjklfsdhjklsdfhildfnifsdnsdfnlfsdnklfsdnkglgfnkl;nmklsdf';
}

$_SESSION['test'] = $myarray;

print("SLEEPING<br />");
sleep(5);
?>


Because you get the SLEEPING output, it appears the high memory consumption is occurring when the script has finished, possibly when the session is trying to be serialized.

I'm running with PHP Version 5.2.1RC4-dev

UPDATE:

Modified the script:
<?php

session_start();
$myarray = array();

$numOfIterations = 1000000;
for($counter = 0; $counter < $numOfIterations; $counter++) {
  $myarray[$counter] =
'jkdfjklsdfjklfsdhjklsdfhildfnifsdnsdfnlfsdnklfsdnkglgfnkl;nmklsdf';
}

$_SESSION['test'] = $myarray;

print("SLEEPING<br />");
print("USAGE: " . memory_get_usage());
sleep(5);
?>

I receive this output:
SLEEPING
USAGE: 160289476
Fatal error: Allowed memory size of 536870912 bytes exhausted (tried to allocate 18874411 bytes) in Unknown on line 0

Right, serializing is expected to consume even more memory, because it has to run malloc/memcpy/free a lot.
Storing 256Mb in session is noway a real task and quite a bad test, because it's not really clear what you're testing and what result you want to get.

If it was performing malloc/memcpy/free, in that order, then it would be freeing the memory back via the free() system call.  Even if it's a bogus test, it's a test to fit your requirements.  Can you please clarify how the session saving is taking more than 3 times the memory usage the script says it's using?

I added some extra debug info to the script:

<?php
	session_start();
	$myarray = array();
	$numOfIterations = 1000000;  // Set to 10000 to see the script succeed
	for($counter = 0; $counter < $numOfIterations; $counter++) {
		$myarray[$counter] =
			'jkdfjklsdfjklfsdhjklsdfhildfnifsdnsdfnlfsdnklfsdnkglgfnkl;nmklsdf';
		if ( ($counter % 100000) == 0) {
			echo("Iterations: $counter, Memory: " . memory_get_usage() . "<br>\n");
		}
	}
	// copy the array, just for kicks
	$_SESSION['test'] = $myarray;
	echo("USAGE: " . memory_get_usage() . "<br>\n");
	echo("Writing Session Data.");
	session_write_close();
	echo("Completed.");
?>

And the output is:

Iterations: 0, Memory: 42336
Iterations: 100000, Memory: 20291200
Iterations: 200000, Memory: 40539976
Iterations: 300000, Memory: 61837196
Iterations: 400000, Memory: 81037264
Iterations: 500000, Memory: 100237332
Iterations: 600000, Memory: 123631704
Iterations: 700000, Memory: 142831772
Iterations: 800000, Memory: 162031840
Iterations: 900000, Memory: 181231908
USAGE: 200431716
Writing Session Data.
Fatal error: Out of memory (allocated 532152320) (tried to allocate 1834294 bytes) in /usr/local/www/apache22/data/foo.php on line 16
Completed.

My environment is FreeBSD 6.2R, Apache 2.2.3, PHP 5.2.0, and  memory_limit = 512M.

>Can you please clarify how the session saving is taking
>more than 3 times the memory usage the script says it's >using?

You have that array in memory and a buffer which constantly grows. To grow the buffer you need to allocate another one, bigger then the previous, to copy data into it and free the small buffer. So in edge case, there will be array + small buf + big buf in the memory in one moment.