In my case this error was caused by a copy of the file ./foo/bar/utility/baz.php appearing in the upgraded location ./foo/bar/baz.php for whatever reason. After removing the latter, the misleading error disappeared. Although I spent several hours staring at small/uppercase pathnames, I initially had no issues with the file.

This can happen for a variety of reasons.

It is importantto pay attention to the error message, it is usuallyvery accuratein pointing out the source of the problem.

Path Case

The most common cause is that, as the error message indicates,Bar.phpthe case of the different components of the pathname does not match the case of the fully qualified class name;

foo/bar/Baz.phpdoes not matchApp\Bar\Baz.

Simply update your application or package so that each path component matches the case of the namespace it owns:

Foo\Bar\Baz.php

Difference between file name and class name or namespace

Check pathnames against namespaces very carefully. Sometimes you name a class (or namespace)FooBar, but its path on disk is "foo-bar", for example. Or maybe just for whatever reason, your namespace doesn't exactly match the pathname of the file.

This will also trigger notifications/warnings. You need to rename the file or rename the class (or namespace).

Generally, it is much easier to change a path or file, because changing a class or namespace name requires refactoring the code to match the new name, while changing a path does not require refactoring anything.

Nested namespaces and missing declarations

Assuming you have:

"autoload": { "psr-4": { "Fizz\Buzz\": "src/" } },

and classDummy, insidedefinitionsrc/Buzz:

// src/Buzz/Dummy.php namespace Fizz\Buzz class Dummy {}

The above method will work, but will throw notifications like other methods. The correct approach is:

// src/Buzz/Dummy.php namespace Fizz\Buzz\Buzz class Dummy {}

You will need to make changes not only to the affected class, but also to any other files that use or import this class. (For example, now declareuse Fizz\Buzz\Buzz\Dummy;).