basic php: why cannot access empty property here? - php

Question

I am have an instance variable w/ a global scope set in a PHP class. The variable gets set successfully in the constructor. But when I got to access in a getter method I get a PHP fatal error. I'm fairly new to php. Why is $sid not getting set?
class Wrapper
{
private $sid = null;
/**
public function __construct($data){
// Retrieve client account automatically
$sid = (isset($data['client_id'])) ? $data['client_id'] : null;
echo("set sid to");
echo($sid);
$this->client = new Account($sid); //this is getting set properly
}
...
public function getSID()
{
return $this->$sid;
}
The code that uses the class looks like this (it is a unit test with PHPunit):
public function testGetSubsctions(){
$clientObject = ClientModel::getInstance(7)->read();
$data = array('client_id' => $clientObject->id);
$this->assertEquals($data['client_id'], 7);
$hello = new Wrapper($data);
$this->assertEquals(7, $hello->getSid());
}
The code above throws the following error:
Cannot access empty property in /path to/wrapper.php on line 243
The code on line 242 is getSid, below
public function getSID()
{
return $this->$sid;
}

Answer 1

in the first part of the code you show, we can see return $this->$sid; but $sid is not defined. non-static property has to be called without the $
The correct syntax is :
public function getSID()
{
return $this->sid;
}
EDIT: see the php documentation for more help on the usage of $this->, static and other things related to OOP in php, and more specifically :
About properties
about static keyword (just for your information to see the difference)

Answer 2

Here is how you set $sid;
public function __construct($data){
// Retrieve client account automatically
$this->sid = (isset($data['client_id'])) ? $data['client_id'] : null;
echo("set sid to");
echo($this->sid);
$this->client = new Account($this->sid); //this is getting set properly
}
The variable is part of the instance so you have to use the $this keyword. Simply using $sid refers to a new local variable.
And also there is no $ sign before sid:
public function getSID()
{
return $this->sid;
}