How to replace a class with a custom class in php?

Given an external dependency within a [email protected] project (without composer):

interface Contract
{
    public static function getFoo(): string;
}

class ExternalDependency implements Contract
{

    public static function getFoo(): string
    {
        return 'gnarf!';
    }
}

That ExternalDependecy is used in dozen other services like so:

class Service extends ExternalDependency {
}

I want to replace ExternalDependency while changing as little code as possible.

I thought I could make use of a class_alias:

class MyFix implements Contract {
    public static function getFoo(): string {
        return 'Foo';
    }
}

class_alias(MyFix::class, SomeExternalDependency::class);

class Service extends SomeExternalDependency {
}

Yet this will issue a warning:

PHP Warning:  Cannot declare class SomeExternalDependency, because the name is already in use in test.php on line 23

In my contrived example, I could delete the ExternalDependency, then it would work as expected. Yet in my actual use-case, the ExternalDependency is a vendor file that I cannot delete.

How to replace the ExternalDependency with MyFix then?

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Example on onlinephp.io

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This question is related, yet all its answers are based on extending the to be replaced class or or lack detail.