Take the following code example:

<?php

class Foo
{
    public function __construct(
        public readonly string $string
    )
    {
    }

    public function strtoupper(): Foo
    {
        return new Foo(strtoupper($this->string));
    }

    public function concat(string $toConcat): Foo
    {
        return new Foo($this->string.$toConcat);
    }
}

$foo = (new Foo('a'))
    ->strtoupper()
    ->concat('b')
    ->concat('c')
    ;

And try to debug this. step_over won't work.