Here is an entire script:

# php x.php

Warning: Cannot use a scalar value as an array in x.php on line 30


<?php

class sw {

    public function stream_open($path, $mode, $options, &$opened_path) {
        return true;
    }

    public function stream_read($count) {
		return FALSE;
    }

    public function stream_eof() {
        return true;
    }

	public function stream_stat() {
		return array(
            'atime' => $this->undefined,
        );

	}

}
stream_wrapper_register('sx', 'sw') or die('failed');

file_get_contents('sx://test');

unset($s);
$s[] = 1; //  Cannot use a scalar value as an array

print_r($s);

?

Fixed in trunk and 5.3.