That might be because of sub-components (Extensions, Framework) incompatibility. However, you must not be concerned about that.
To install the latest version, do the following two commands:
> pear config-set auto_discover 1
> pear upgrade
This will take a while, and will update all that is presently installed.
As you have some extensions already offered and the latest version of PHPUnit would possibly need those, so update them to prevent failure in the next command:
> pear install pear.phpunit.de/PHPUnit
This should now install it:
Did not download optional dependencies: phpunit/PHP_Invoker, use --alldeps to download automatically
phpunit/PHPUnit can optionally use package "phpunit/PHP_Invoker" (version >= 1.0.0)
downloading PHPUnit-3.6.7.tgz ...
Starting to download PHPUnit-3.6.7.tgz (118,349 bytes)
..........................done: 118,349 bytes
install ok: channel://pear.phpunit.de/PHPUnit-3.6.7
As you reported in comments, this did not work out.
Probably pear is in a state that it doesn't understand where top and bottom is.
Time to force. First, clear the cache:
> pear clear-cache
reading directory C:\...\Tmp\pear\cache
0 cache entries cleared
Then force channel updates:
> pear channel-update -f pear.php.net
Updating channel "pear.php.net"
Update of Channel "pear.php.net" succeeded
> pear channel-update -f pear.phpunit.de
Updating channel "pear.phpunit.de"
Update of Channel "pear.phpunit.de" succeeded
Then force the PHPUnit install:
> pear install -a -f phpunit/PHPUnit