This bug exists in 4.3.0 and 4.3.1 but *not* in 4.2.3. I installed 4.2.3 over 4.3.1. 

./configure --with-ldap --with-mysql --with-apxs

<?
session_start();
if ( session_is_registered( "foo" )) echo $_SESSION["foo"];
$_SESSION["foo"] = "bar";
?>

Problem: session_start() generates the error

session_start(): read returned less bytes than requested in /var/www/session.php on line 2

the 2nd++ time the page is loaded.

- If no Session variable is set there's no error
- /tmp/sess_.. looks fine:
    foo|s:3:"bar";


PHP is the only 'custom' software, all the other apps are part of Debian stable. The box is a PowerMac (ppc).