Derick Rethans has a post to his site covering some optimizations around PHP's "switch" handling and how it has changed in the upcoming PHP 7.2 release.

PHP 7.2 is around the corner soon, and comes with many optimisations. Many new optimisations are implemented in opcache, but some others are implemented in PHP itself. One optimisation that falls in the latter category is an optimisation of the switch/case construct.

He then gets into the differences between the previous functionality and how it has changed, mostly in how the language handles the evaluation of the "case" statements. He include a flow diagram of how the pre-7.2 flow happened and how, with PHP 7.2, a "jump table" is used to optimize the process. This table allows PHP to perform a lookup on the value rather than evaluating equality (like in an if) on string values. If it's not a string the same evaluation happens as before, however.