Callable and callbacks.

<?php
class CurrentTime {
    public function __invoke($format) {
        return 'Invoked : ' . date($format);
    }

    public static function getCurrentDateTime($format) {
        return 'Func array : ' . date($format);
    }
}

$invokable = new CurrentTime();

$function = function($format) {
    return 'Closure : ' . date($format);
};

$funcarray = array('CurrentTime', 'getCurrentDateTime');

function examineCallbacks($callbacktype, $callback) {
    echo gettype($callback), ' is ', (!is_callable($callback) ? 'not ' : ''), 'callable', PHP_EOL;
    if (is_callable($callback) && !is_array($callback)) {
        echo $callback('r'), PHP_EOL;

    } else if (is_array($callback)) {
        echo 'Handling ', $callbacktype, ' via call_user_func', PHP_EOL, call_user_func($callback, 'r'), PHP_EOL;
    }
}

$string = 'date';

examineCallbacks('Invokable', $invokable);
examineCallbacks('Closure', $function);
examineCallbacks('String', $string);

examineCallbacks('Array', $funcarray);