PHP函数只返回第一个结果

I'm trying to create a simple function to return only the first set of results from a query but I'm getting the following error:

Parse error: syntax error, unexpected '[' in /Users/MAMP_SITES/website/classes/DB.php on line 78 

Here is the code that causes the error:

$user = DB::getInstance()->get('users', array('username', '=', 'rich'));

if(!$user->count()) {
    echo 'No user';
} else {
foreach($user->results() as $user) {
    echo $user->first()->username;
}
}

Here are the relevant functions from DB.php:

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

public function first() {
    return $this->results()[0]; // This is line 78 where the error is
}

I'm getting this code from a tutorial that may be a little out of date.

Array dereferencing is only available in more recent PHP versions. To return the first array item you can use the current() function.

return current($this->results());

You can try:

public function first() {
    $results = $this->results();
    return (sizeof($results)>0 ? $results[0] : null);
}

OR

public function first() {
    return current( $this->results() );
}