I had the same problem until I stumbled across this site:
https://blog.hypercubed.com/index.php/archives/2005/12/26/wordpress-20-fix/
I replaced the first 25 lines of code as he suggested and whamo, my plugins now activate. just be sure to grab that last line of code with the ending bracket }. I missed it the first time and it returned many more errors than what I originally had.
