Paul Jones has posted some of his opinions on unit testing in the PHP community - or lack there of.
Without having done actual research, and depending on my personal experience alone, I would assert that in PHP userland there are maybe 20 or more general-purpose CMSes, 50 or more public frameworks, easily 100 or more templating libraries and only 5 unit-test systems.
His point is that there seems to be a glut of other application types - the CMSes, frameworks, etc - and not enough unit testing apps to spread around. He also comments of how this can affect the spread of unit testing in the PHP community.
Be sure to check out the long list of comments on the post with other opinions including one that's repeated - "unit testing is low level while those other things mentioned (CMSes, frameworks, etc) are high level). you can't really compare them."