Writing extensions is fun, but it's not as fun as hacking PHP. So, we're going to focus on hacking, we're going to imagine that we are introducing some new language feature, by RFC.
Without focusing on the RFC process itself, you need to know which are the relevant parts of PHP you need to change, in order to introduce new language features. You also need to know how PHP 7 works, about each stage of turning text into Zend opcodes.
After talking a bit about some of his thoughts and troubles with screencasting in general he looks at "The Beginning" of PHP's translation from text to functionality: the lexing. He introduces the basic concept around how a lexer works and how it migrates the pieces over to tokens. He then starts in on the parsing of these tokens and, finally, the AST (abstract syntax tree) resulting from the combination of these pieces, executed against a piece of code.
With that out of the way, he starts in about the "hack" - a hipster expression that only works with strings and throws an exception otherwise. He shows the pieces he had to edit to create this new expression and it's matching token/AST node.