PHP Error: Cannot access private property

I have a class like this:

class Session {
    private Session_Database $db;

    public function __construct(){
        // Instantiate new Database object
        $this->db = new Session_Database();

        // Set handler to overide SESSION
        session_set_save_handler(
            [$this, '_open'],
            [$this, '_close'],
            [$this, '_read'],
            [$this, '_write'],
            [$this, '_destroy'],
            [$this, '_gc']
        );

        // Start the session
        session_start();
    }

    /* ... */

    public function _close(): bool {
        // Close the database connection
        $this->db->close();
        return true;
    }

    /* ... */
}

I'm seeing this error in the logs:

PHP Fatal error: Uncaught Error: Cannot access private property Session::$db in [line number of $this->db->close()]

Stack trace:
#0 [internal function]: Session->_close()
#1 {main}\n thrown in ...

However when I test the code by stepping through in xdebug it works as expected.

What is going on?

As far as your question, "what is going on?"

This appears to happen if memory is exhausted during the course of a PDO query. I believe it has something to do with the memory used by PDO Buffered queries and the way the PHP internal code frees up memory after memory is exhausted.

I can reproduce the condition you have experienced with the following code, (note: this is using register_shutdown_function. Both the session_close code and the register_shutdown_function will run after a memory exhausted error):

<?php

class TestingClass
{
    private int $something_private;

    public function getThisDone(): void
    {
        $this->something_private = 0;
    }
}

// get a reference to the testing class
$obj = new TestingClass();

register_shutdown_function(fn() => $obj->getThisDone());

// get a pdo connection and run a query that returns a lot of data
$pdoConnection = get_db_read_connection();
$pdoStatement = $pdoConnection->prepare('SELECT * FROM your_table_with_a_lot_of_records LIMIT 100000');
$results = $pdoStatement->fetchAll(PDO::FETCH_CLASS, '\stdClass');

And the resulting fatal error:

Note:

I cannot reproduce the error if I simply trigger an out of memory error with string concatenation, like below.

// use code above but replace pdo connection and query with this
while(true)
{
    $data .= str_repeat('#', PHP_INT_MAX);
}

As far as "why is this happening?"

I don't think it should be. It seems like this is a good candidate for a PHP bug report.