The pecl-radius package rightly has php_targets="5.3" only. However, a patch is floating around to make it cope with PHP 5.4 that annoyingly has still not made it into a proper release. Would it be possible to have this in the tree as 1.2.5-r2 perhaps?
I've updated the package to -r2 and checked that php-5.{3,4} cli reports radius as enabled. If there's fallout from this, just assign it to me.