Post character limit(24998) after switch to PHP5

[Idealist]

Ever since switching from PHP 4 to 5 new posts and edits that aren't done via quick edit end up blank if they're more than 24998 characters. I've had to place a limit on post size via the "Maximum Characters Per Post" setting, so users won't lose edits or new posts.

Here's a bit of info about the forum/server:
CENTOS Enterprise 4.4 i686
Apache 1.3.39
PHP 5.2.4
MySQL 4.0.27
vB 3.5.4 with security patches

There've been no errors reported by PHP or in the server's error_log. I've tried on a test instance of vB without any plugins and the same happens. Any ideas?