Nit: these are called "untagged" union types, as opposed to much more common "tagged" union types (aka algebraic data types or ADTs) where each variant is named

Nit: these are called "untagged" union types, as opposed to much more common "tagged" union types (aka algebraic data types or ADTs) where each variant is named and distinct from each other even when types are same. Untagged union types are uncommon mainly because it complicates a type checker for little reason, and most frequently found where types were added as an afterthought and the base language couldn't be changed. In the case of PHP though, it doesn't have structural typing (unlike TypeScript) and every variant should be nominal or primitive types, so this is of the least concern.