在PHP对象中定义变量时出错

Question

I am getting the following error in the following code:

Class primeField implements field {
    private $intmodulus = '';
    public function generator(){
        return ;
    }

    public function modulus(){
        return $this->$intmodulus;
    }
    public function __construct($modulus , $base=0) {
        if (is_resource($modulus) && get_resource_type($modulus) == "GMP integer"){
            $this->$intmodulus = $modulus;
        } else{
            $this->$intmodulus = gmp_init($modulus , $base); \\line 70
        }
    }
}
$a = new primeField(11);
$a->modulus();

Notice: Undefined variable: intmodulus in /Users/admin/PHP ECC/finitefield.php on line 70 Fatal error: Cannot access empty property in /Users/admin/PHP ECC/finitefield.php on line 70

Why

Answer 1

The syntax is

$this->intmodulus 

not $this->$intmodulus.

You get an error saying "cannot access empty property" because $intmodulus is undefined and hence accessing it gives NULL. The NULL gets converted into an empty string when you attempt to use it as a property name.

If the value of $intmodulus was the name of a valid property (e.g. if $intmodulus == "intmodulus"), you would be accessing the property with that name.