Laravel - A PHP Framework For Web Artisans
Bludit - Bludit is a web application to build your own website or blog in seconds, it's completely free and open source. Markdown support.
CodeIgniter - A Fully Baked PHP Framework
Grav - The modern open source flat-file CMS
Yii Framework - Yii is a high-performance component-based PHP framework best for Web 2.0 development.
WonderCMS - WonderCMS is the smallest flat file CMS. Completely free, open source, privacy oriented for over 12 years.