I write my software with Haskell, NixOS, comprehensive testing, static analysis, linting, a great deal of care, etc.
If I had written everything in one big PHP file however, the difference to a paying customer would be exactly nil.
Only thing 1 file can't do is allow concurrent work if bad version control is used. If patches are tight with good patch theory, 1 file is fine again.
Modern programming languages have namespaces, catch type bugs, all of it works quite well with 1 file. Recent OAuth Cloudflare 1 file library (coded a bit with AI) is a breath of fresh air, 2600 lines of data types and behavior in one place.