What kind of PHP syntax is this?

Can you please tell me what kind of PHP syntax is:

echo Html::{'div.my-class#my-id'}('This is element content')

?

It's from https://github.com/decodelabs/tagged library.

I mean I know what it does, but I don't understand the syntax used and why it works :) (braces immediately after the scope operator). Looks like they're using braces to 'generate' the name of the function to call, but - on the other hand - it contains characters like '.' and '#" and variable parameters ('my-class', 'my-id'). I'm confused...

I tried googling for over 2 hours, but no luck :/

let's reconstruct this from first principle

1. you can use {value} to put a dynamic method/attribute name

i.e if you have

class Dog {
    public string $name = 'doggy';
    public function walk() {
    }
}

you can do

$dog = new Dog();
$dog->walk(); 
$dog->{'walk'}(); // equivalent 

echo $dog->name;
echo $dog->{'name'};

as you can do this and 'walk' and name are value, they can also come from variables

$attribute = 'name';
$dog->{$attribute};

2. PHP has some magic methods to treat undefined method / attributes , the full list is there https://www.php.net/manual/en/language.oop5.overloading.php

for example __callStatic() will be called whenever you call a static method for which the name is undefined

like this

class Html
{
    static public function __callStatic($name, $arguments)
    {
        // Note: value of $name is case sensitive.
        echo "Calling object method '$name' "
             . implode(', ', $arguments). "\n";
    }
    
};

Html::iDontExist();

will output Calling object method 'iDontExist'

3. now if we mix both 1 and 2 we can do

Html::{"Hello world"};

It will output Calling object method 'Hello World'

Conclusion

1. using {} syntax you can have method that don't follow normal convention (i.e they can have spaces, emoji, special caracters etc. )
2. using overloading magic method like __callStatic you can have methods that don't exists before hand

so by combining 1 and 2 your library is allowing a method to already be an argument by itself and all the logic of treating this name will be in a __callSatic

so it would be the same as doing Html::something('div.my-class#my-id', 'This is element content' ) if something was a defined static method