When people give talks on the "S.O.L.I.D." design principles one of the letters that doesn't get enough attention is the "i" - the "interface segregation principle". This talk seeks to redress that imbalance by going into a bit more in-depth into: * An introduction to interface segregation and an explanation of how it make your code easier to test. * Why in PHP we need to apply the principle more broadly, to make types be more specific, so that code is more reasonable. * Me babbling on about emotions, and how good code is boring. Which is good!