致命错误：不在对象上下文中时使用$ this-PHP，PDO，会话，

Question

I'm trying to modify a script that stores my session data in a database rather than the default but I'm having a bit of trouble as I keep getting the error above. It says I'm getting the error on lines 176 and 201.

Here is the function inside the sessionPdo class

public function _write($id, $sessionData)
{
    $session = $this->fetchSession($id); // line 176
    if( $session === false ) {
        $stmt = $this->db->prepare('INSERT INTO sessions (id, data, unixtime) VALUES (:id, :data, :time)');
    } else {
        $stmt = $this->db->prepare('UPDATE sessions SET data = :data, unixtime = :time WHERE id = :id');
    }
$stmt->execute( array(
        ':id' => $id,
        ':data' => $sessionData,
        ':time' => time()
    ));

if( $this->transaction ) {
    $this->db->commit();
}
}

And here is the fetchSession method in case it helps.

public function fetchSession($id)
{
    $stmt = $this->db->prepare('SELECT id, data FROM sessions WHERE id = :id AND unixtime > :unixtime');
    $stmt->execute( array(':id' => $id, ':unixtime' => ( time() - (int) ini_get('session.gc_maxlifetime') ) ) );
    $sessions = $stmt->fetchAll();

    if( $this->transaction ) {
        $this->db->commit();
}

    return empty($sessions) ? false : $sessions[0] ;
}

I've struggled to find what I'm doing wrong as all the solutions I've found are because the programmer tried to call $this from outside of the class. I'm new to PDO so thanks in advance for any help you can give.

Answer 1

You are calling function _write statically, that means somewhere in your code you can find:

sessionPdo::_write($id, $sessionData);

What you have to do is to create object of sessionPdo class and then run this function on this object, ie:

$sessObj = new sessionPdo();
$sessObj->_write($id, $sessionData);

Answer 2

Yeah, the problem is that you are calling _write() statically. You need to read up on the difference between static and instance methods. I apologize for not explaining it here but there are probably 5,000 people on SO alone who have run into the same problem, which is solved with about 15 minutes of applied reading.

When you're comfortable with that, research the Singleton and Factory patterns, which are two ways of providing access to object instances from a static context and are commonly applied in this situation.