This is an odd mix of common sense, random cosmetic advice, subjective bits and points that are plain wrong or contradictory to other points. For example:

    Bad:
        $ymdstr = $moment->format('y-m-d');

    Good:
        $currentDate = $moment->format('y-m-d');

Both are inferior to "$ymd".

    Use searchable names
    
        addExpireAt(DateGlobal::SECONDS_IN_A_DAY);

Who would ever need to search for that? You'd search for "Expire". This doesn't preclude from using named constants, but it has nothing to do with search-ability per se.

    Don't use flags as function parameters

    Good:

        function createFile($name) { touch($name); }
        function createTempFile($name) { touch('./temp/'.$name); }

Now, this is how you create the good old copypasta.

What you need instead is a single universal function that does in fact accept a boatload of parameters, but that is never meant to be called directly. You then supplement it with a host of simpler one-liners that fill in most of the parameters with defaults and forward just the select few.

    Avoid negative conditionals

should be "Avoid double-negative conditionals".

    Avoid conditionals

this is just overly broad to the point of being completely pointless. What the example proposes is to use abstraction and inheritance instead. In some cases it helps, but it many it leads to needlessly obscure code that's impossible to read off the page.

    Use method chaining

    Good:

        $car = (new Car())
          ->setColor('pink')
          ->setMake('Ford')
          ->setModel('F-150')
          ->dump();

Except at some point you will need setModel() to fail for some arguments, e.g. setModel('Trabant'), and you will end up with some methods returning $this and some not. This is not going to be a "clean code" by any measure.

(edited for clarity a bit)