php static variable initialization using the $this keyword - php

Question

I'm having a method inside a class, in which I want to initialize a static variable.
1/ when I initialize the variable, then affect it to some other value using the $this keyword, it works. E.g.:
class Test {
// ...
function test($input_variable)
{
static $my_static_variable = null;
if (!isset($my_static_variable))
$my_static_variable = $this->someFunction($input_variable);
// ... some further processing
}
}
2/ However, when I try to directly initialize / construct the variable with the $this keyword, there is a syntax error : unexpected '$this' (T_VARIABLE):
class Test {
// ...
function test($input_variable)
{
static $my_static_variable = $this->someFunction($input_variable); // *** syntax error, unexpected '$this' (T_VARIABLE)
// ... some further processing
}
}
Is 1/ a good way to initialize a static variable?
Why is 2/ not allowed, as it is supposed to do the exact same thing than in 1/?
I'm using PHP 5.5.21 (cli) (built: Jul 22 2016 08:31:09).
Thanks

Answer 1

You can't use $this on a static variable. You can use self with scope resolution operator (::) instead.
Here is the example:
class Foo {
public static $my_stat;
static function init() {
self::$my_stat= 'My val';
}
}

Answer 2

Static variable and function can be accessed without instantiation of the class. You can not use $this to access the variable or function declared as static.
You have to use scope resolution operator:: to access variable and functions declared as static.
For variables:-
class A
{
static $my_static = 'foo';
public function staticValue() {
return self::$my_static;// Try to use $this here insted of self:: you will get error
}
}
class B extends A
{
public function fooStatic() {
return parent::$my_static;
}
}
Use to the following to access the variable:-
print A::$my_static
For functions:-
class A {
public static function aStaticMethod() {
// ...
}
}
You can call the function as follows:-
A::aStaticMethod();

Answer 3

I think I have the answer. In the php documentation, it is stated the following:
Static variables may be declared as seen in the examples above. From
PHP 5.6 you can assign values to these variables which are the result
of expressions, but you can't use any function here, what will cause a
parse error.
So I guess this also holds for PHP 5.5.
As #MagnusEriksson pointed out, I could have used a class property as well. However, I don't want my variable to be accessed somewhere else apart from the test() method.
BTW, somehow the same thing is stated for static properties in the doc:
Static properties cannot be accessed through the object using the
arrow operator ->.
Like any other PHP static variable, static properties may only be
initialized using a literal or constant before PHP 5.6; expressions
are not allowed. In PHP 5.6 and later, the same rules apply as const
expressions: some limited expressions are possible, provided they can
be evaluated at compile time.