Assuming that Zend Optimizer was working in your previous version of PHP, the needed tweaks are likely already in your php.ini and you already have the install files sitting somewhere on your serever...
cp /path/to/ZendOptimizer-2.1.0-Linux_glibc21-i386/data/4_3_0_comp/ZendOptimizer.so /usr/local/Zend/lib/
And restart Apache via your favorite method.
Ta da. The 4.3.0 works fine with 4.3.1 per Zend and has had no problems with phpManager here.
- Jason