I should probably narrow the query a bit (I'm using a job board aggregator), but there are a number of startups built on PHP, most notably Facebook. Yahoo uses it in places, and don't forget all the companies who need a WordPress, Magento, Drupal, etc. developer. These technologies run multi-million dollar businesses. And after years of consulting, I can tell you business owners don't care what's under the hood if it solves their problem with good ROI.