php访问类属性数组，错误不是数组

Question

Trying to get the values in an array but returning an error:

Warning: Invalid argument supplied for foreach()

and when using array_key_exists() :

Warning: array_key_exists() expects parameter 2 to be array

Why is the property array $credentials not recognized as an array here?

class Config {
   private $credentials = array(
        'host' => 'localhost',
        'dbname' => 'testdb',
        'user' => 'username',
        'pass' => 'password'
    );

    public static function get($credential) {

        if(array_key_exists($credential, $credentials)) {

            foreach($credentials as $key => $value) {

                if($key === $credential) {
                    return $credentials[$key];
                }
            }
            return false;
        } else { 
            echo 'Credential does not exist.'; 
        }
    }
}


$test = new Config();

echo $test->get('host');

Answer 1

private $credentials 

declares an instance variable. You reference $credentials as a local variable. If you would want to reference the instance variable, you would have to use the notation $<object>->variableName. In an instance-method, you could use $this->credentials. But, you are in a static function, which does not have an associated object. Therefore, you cannot reference an instance variable.

In your calling function you would have to have a reference to an object of class Config (let's say $myConfig) and call a method of the name "get", like

$myConfig->get($credential);

"get" would need to be declared a method and not a static function. Then the method "get" could access the instance variable "credentials" as $this->credentials.