Cause of the error: “Using $this when not in object context protected function”

Question

I get the following error when using the following class in my code:

Using $this when not in object context protected function

Here is my code:

<?php
class Sign {
    private $db;
    private $errors = array();
    private $passed = true;

    public function __construct() {
        $this->db = DB::instance();
    }

    public static function sign($alias, $password) {
        if(!self::exists($alias)) {
            self::signup($alias, $password);
        } else {
            self::signin($alias, $password);
        }
    }

    protected function exists($alias) {
        $stm = $this->db->prepare('SELECT * FROM users WHERE alias = ?');

        if($stm->execute(array($alias))) {
            return true;
        }

        return false;
    }

    protected function signup($alias, $password) {
        $salt = Hash::salt(32);

        $stm = $this->db->prepare('INSERT INTO users(alias, balance, password, salt, created_at)
                                   VALUES (:alias, :balance, :password, :salt, :created_at)');
        $stm->execute(array(
            ':alias'        => $alias,
            ':balance'      => 0,
            ':password'     => Hash::make($password, $salt),
            ':salt'         => $salt,
            ':created_at'   => date('Y-m-d H:i:s')
        ));
    }

    public function errors() {
        return $this->errors;
    }

    public function passed() {
        return $this->passed;
    }
}

Why am I getting this error? Line 20 is inside of protected function exists() where I use $this->db->prepare()

What would cause this to be happening? I tried changing some of the public, protected, etc. to no avail. I had them as private and protected still gives this error.

Answer 1

You can't mix and match static and instance like this. Your real problem comes in how you're calling exists:

public static function sign($alias, $password) {
    if(!self::exists($alias)) {
        self::signup($alias, $password);
    } else {
        self::signin($alias, $password);
    }
}

Unless you really need a static method here, you should probably create an object and call sign() as an instance method of that object.

You could also use sign() as a factory, but as in any programming, there are several ways to $cat->skin().

public static function sign($alias, $password) {
    $retVal = new Sign();
    if(!$retVal->exists($alias)) {
        $retVal->signup($alias, $password);
    } else {
        $retVal->signin($alias, $password);
    }
    return $retVal;
}