A mixed typed represents the absence of type information. It is a union of all types, which means it can be anything. This in turn leads to suboptimal PHPStan analysis results which can lead to missing errors or even false positives.

Markus Staab

A retrospective on years of contributions to PHPStan's type inference engine, focusing on handling mixed types. Covers improvements to type narrowing from conditionals, type casts, and comparisons; support for immediately-invoked function expressions; new PHPDoc types like non-falsy-string and @param-out; and advanced type inference for regex patterns, printf functions, and database queries through AST analysis. Demonstrates how subtractable types help narrow down unknowns by eliminating what types cannot be.

A mixed type PHPStan journey