I was playing with Google Trends on the weekend and was very pleased and a little surprised to see Yii overtake CakePHP and Symfony in 2012. Yii is the youngest of the bunch and it is very nice to see it doing so well. It looks like CodeIgniter and Yii are on a similar trend.
I compared cakephp, codeigniter, yii, symfony
I excluded Zend Framework as I was not sure what would be the best search phrase to use. If you include just the word Zend it comes above all the others.
I understand this is a very superficial evaluation of popularity and does not take into account any merit or quality, but I still think its worth taking note of.