wordpress: how to set up a custom URL AJAX API

I’m trying to implement a custom JSON API upon WordPress, like /api3/get_posts, I refer to the implementation of plugin JSON-API, but it just doesn’t work. Below is my code:

$myPluginBase = 'api3';

function my_plugin_init() {
    global $wp_rewrite;
    add_filter('rewrite_rules_array', 'my_plugin_rewrites');
    add_action('template_redirect', 'my_plugin_template_rewrite');
    $wp_rewrite->flush_rules();
}

function my_plugin_template_rewrite(){
    global $myPluginBase;
    if( isset( $_REQUEST[ $myPluginBase ] ) ){
        echo $_REQUEST[ $myPluginBase ];
        exit;
    }
}

function my_plugin_activation() {
    // Add the rewrite rule on activation
    global $wp_rewrite;
    add_filter('rewrite_rules_array', 'my_plugin_rewrites');
    $wp_rewrite->flush_rules();
}

function my_plugin_deactivation() {
    // Remove the rewrite rule on deactivation
    global $wp_rewrite;
    $wp_rewrite->flush_rules();
}

function my_plugin_rewrites($wp_rules) {
    global $myPluginBase;
    if (empty($base)) {
        return $wp_rules;
    }
    $my_plugin_rules = array(
        "$myPluginBase$" => "index.php?{$myPluginBase}=info",
        "$myPluginBase/(.+)$" => "index.php?{$myPluginBase}=$matches[1]"
    );
    return array_merge($my_plugin_rules, $wp_rules);
}

// Add initialization and activation hooks
add_action('init', 'my_plugin_init');
register_activation_hook( __FILE__, 'my_plugin_activation');
register_deactivation_hook( __FILE__, 'my_plugin_deactivation');

It seems like the template_redirect action works, as I open /api3=hello, the server will send hello back. But if I try to open /api3/hello, it keeps showing me the home page.