Question

I need to have a variable that only one function can write (let’s call that function a) and that only one other function can read (let’s call that function b). Is that possible?

Answer 1

You could use a static variable:

function foo($val=null) {
    static $var = null;
    if (!is_null($var)) $var = $val;
    return $val;
}

Here $var is only visible inside the function foo and is maintained throughout multiple calls:

foo(123);
echo foo();  // 123
foo(456);
echo foo();  // 456

Or use a class with a private member and access/modify it with public methods:

class A {
    private $var;
    public function setVar($val) {
        $this->var = $val;
    }
    public function getVar() {
        return $this->var;
    }
}

With this the private member var is only visible to a particular instance of this class:

$obj1 = new A();
$obj1->setVar(123);
$obj2 = new A();
$obj2->setVar(456);
echo $obj1->getVar();  // 123
echo $obj2->getVar();  // 456

If you make the member static, then there is just one for the class instead of for each instance:

class A {
    private static $var;
    public function setVar($val) {
        self::$var = $val;
    }
    public function getVar() {
        return self::$var;
    }
}
$obj1 = new A();
$obj1->setVar(123);
$obj2 = new A();
$obj2->setVar(456);
echo $obj1->getVar();  // 456
echo $obj2->getVar();  // 456