When I upgraded the recaptcha plugin to 3.1.3 it fubared my WP giving me the error discussed in the first post of this thread. I could not even get to the Dashboard anymore to disable it. Instead of deleting the wp-recaptcha directory, I moved it out of the plugins directory: mv wp-recaptcha .. Once it was no longer in the plugins directory I could get to the WP Dashboard. WP noticed it was no longer in the plugins directory and disabled it. Once I completed my upgrade to PHP5, I moved the wp-recaptcha directory back to the plugins directory. mv ../wp-recaptcha . Once it was moved back, it was listed again in the WP Dashboard as disabled. I enabled it and it worked fine.
Hmmm… interesting. The only other thing I did was to move the wp-recaptcha directory out of the plugins directory to let WP think it was deleted and disable it. Once the upgrade was done I put the wp-recaptcha directory back and enabled it from the Dashboard and it worked.
I had this same problem. You need PHP5.
