How to create or define class in PHP at runtime?

You could, could, use eval.
But please, don't!

Meta-programming is not something PHP is particularly good at, so you should try to solve your problem without creating classes at runtime.

you could create a generic extendable class and use the magic __set() and __get() methods to dynamically add properties. no methods though, sorry.

No.

You can however, add properties dynamically to particular instances.

Saying that eval permits this is like saying in Java you could, problematically, write a source file, compile it, and load it (or, for that matter in PHP you could include it). Sure, it's possible, but it's not actually part of the language.

basically

 class A {
     function B() {...}
 }

everything in php is defined at runtime, so you should be fine with that. You can also try

  include 'some.class.php';

"include" is the same as "eval" but somehow has a better image.

Since PHP7.0 it's possible to use Anonymous classes for it. Example:

$request = new class (
    field1: $this->faker->word(),
    field2: $this->faker->word(),
    field3: $this->faker->word(),
    field4: $this->faker->word(),
    field5: $this->faker->word(),
) implements RequestInterface {
    private string $signature;

    public function __construct(
        public string $field1,
        public string $field2,
        public string $field3,
        public string $field4,
        public string $field5,
    ) {
    }

    public function getSignature(): string
    {
        return $this->signature;
    }

    public function sign(string $signature): void
    {
        $this->signature = $signature;
    }
};