mutable php and python

"Mutable" means "changeable."  In PHP all variables of the main scope are mutable (IMHO a great shortcoming of the language).  Inside a function, you have a separate scope that gives you the ability to make these variables immutable to the outside code.  Except for the superglobal variables and variables declared "global."  Inside a class definition, the visibility and therefore the mutability of the variables is controlled by the public, protected and private declarations, with the default (unfortunately) being public.  

After that, you might want to learn about namespaces.  Start your adventures here:
http://php.net/manual/en/language.namespaces.php

main scope are mutable

is this the global scope

or the local scope in a method


which means that the other is not mutable?

By the "main scope" I mean any variable that is defined outside of a function or method.  I think that is the same as Global Scope in PHP.
http://php.net/manual/en/language.variables.scope.php

So all global scope variables can be modified in a method and outside the class

But local scope variables can only be modified in the method?

Yes, that's apparently correct.  Example here, but nobody would write code like that.  The goal of variable scope is to ensure that your variables are not available for reading or writing outside a sensible context.  This reduces the risk of variable name collisions and enables two or more programmers to work on the same project simultaneously.  The existence of the global scope is an artifact of the early history of PHP, when computer science took a back seat to ease of use.

<?php // demo/temp_rgb192.php
error_reporting(E_ALL);

Class Thing
{
    public function __construct()
    {
        global $a;
        $a = 3;
    }
}

$x = new Thing;
var_dump($a); // PRINTS "int(3)"

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Also, be aware of object properties which have their own visibility according to the rules of public, protected and private.  Unlike global vs functional scope, visibility errors trigger noticeable failures, making it much easier to detect and correct malformed code.

https://www.experts-exchange.com/viewCodeSnippet.jsp?refID=40212835&rtid=20&icsi=1

so you created an example where the local variable line9 is actually a global variable
and because line9 is now global, it is mutable?

Yes. Try this, for example.  But please don't think this is good coding practice -- it is not.  I'm only proving a point to address your question; this is not something you want to do!

<?php // demo/temp_rgb192.php
error_reporting(E_ALL);

Class Thing
{
    public function __construct()
    {
        global $a;
        $a = 3;
    }
    public function get_a()
    {
        global $a;
        return $a;
    }
}

$x = new Thing;
var_dump($a); // PRINTS "int(3)"

// MUTATE $a
$a = 4;
$b = $x->get_a();
var_dump($b); // PRINTS "int(4)"

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public function get_a()
    {
        global $a;
        return $a;
    }

so a must be called global again in order to have value of 4?

so
$this->a
is always local?


could we do
global $this->a
$this->a=3

$this->a is not really a "local" because the object-oriented syntax does not distinguish between private, protected, and public.  They are all called $this-> something.  If you omit $this-> you get a local variable, one defined only in the scope of the method.  Unless you use the global keyword, which nobody would ever want to do inside a class method.

I have never tried it (feel free to make your own experiment) but global $this->a should be a parse error.

It's not a term limited to any specific programming language; it's an industry concept.  Here are the dictionary definitions:
http://dictionary.reference.com/browse/mutable?s=t
http://dictionary.reference.com/browse/immutable?s=t

Here is some discussion on the concept.
http://en.wikipedia.org/wiki/Immutable_object

Here is a PHP example:
http://php.net/manual/en/class.datetimeimmutable.php

Local and global examples are good because it is easier to change global

if the method designates it then the x variable can be immutable, and can NOT be valued changed.

Please understand that python represents all its data as objects (unlike php), , so it was kind of necessary to have the terms mutable and immutable

So now I think of python data as php objects

Thanks.

I have a related followup question about the answer.

https://www.experts-exchange.com/questions/28482748/variables-objects-passed-by-reference-value.html