A fully qualified function name is a little bit faster than a non-qualified one. Toon Verwerft explains it all in his lastest blogpost.

Today, a little conversation on Twitter escalated rather quickly. Apparently PHP runs function calls differently depending on namespaced or non namespaced context. When calling functions in a namespaced context, additional actions are triggered in PHP which result in slower execution. In this article, I'll explain what happens and how you can speed up your application.

http://veewee.github.io/blog/optimizing-php-performance-by-fq-function-calls/