Most (proprietary) 25 year old PHP codebases I've seen are a huge mess riddled with issues, exuberant loc, mix of tabs and spaces and weird indentation, dry violations, slightly diverging code blocks copy-pasted all over the place, etc., etc. Resolving technical debt (let alone reviewing the "stuff that works" like SQL queries) is often low priority because it's tedious and does not create any "business value".