Why the two releases so close in time? The OCI8 extension is one of the few extensions that has a dual identity, being in both the PHP bundle and in the PECL repository. OCI8 1.4.2 contains the same code as in the recent PHP 5.3.3 release. The PECL bundle was made for anyone who wants a PHP 5.5.3-identical version of OCI8 when installing via PECL.
PECL OCI8 1.4.3 has one extra fix (bug #51610) that came too late in the PHP 5.3.3 release cycle and was deferred. It has now been merged to PHP's source code system and will also be in a subsequent PHP distribution e.g. PHP 5.3.4 or later. The bug impacts Oracle users who run PHP OCI8 1.3+ using Oracle 10.2.0.4+ client libraries and who use PHP as a command line scripting language. It causes a time delay at PHP process termination. Sites invoking PHP via a web server are unlikely to have noticed any issue. The "problem" had existed for several years but suddenly became "hot" - perhaps when users started to upgrade to PHP 5.3 which was the first full PHP release to include the affected code. I was surprised how many people are using PHP for command line processing. The OCI8 fix requires an Oracle client library patch for Oracle bug 9891199 before it will have an effect on PHP. There are several alternative PHP fixes postulated in bug #51610 that don't require an Oracle library patch, but they have limitations (no DRCP support; potential memory leak in far edge cases) and haven't been tested.
The OCI8 extension can be installed on PHP 4.3.9 onwards. The PHP community would strongly suggest that anyone still using PHP 4 should upgrade to PHP 5 for overall stability and security reasons. PHP 5.2 or PHP 5.3 are preferred.