Is it possible to declare a method static and nonstatic in PHP?

You can do this, but it's a bit tricky. You have to do it with overloading: the __call and __callStatic magic methods.

class test {
    private $text;
    public static function instance() {
        return new test();
    }

    public function setText($text) {
        $this->text = $text;
        return $this;
    }

    public function sendObject() {
        self::send($this->text);
    }

    public static function sendText($text) {
        // send something
    }

    public function __call($name, $arguments) {
        if ($name === 'send') {
            call_user_func(array($this, 'sendObject'));
        }
    }

    public static function __callStatic($name, $arguments) {
        if ($name === 'send') {
            call_user_func(array('test', 'sendText'), $arguments[0]);
        }
    }
}

This isn't an ideal solution, as it makes your code harder to follow, but it will work, provided you have PHP >= 5.3.

I would make a hidden class as the constructor and return that hidden class inside the parent class that has static methods equal to the hidden class methods:

// Parent class

class Hook {

    protected static $hooks = [];

    public function __construct() {
        return new __Hook();
    }

    public static function on($event, $fn) {
        self::$hooks[$event][] = $fn;
    }

}


// Hidden class

class __Hook {

    protected $hooks = [];

    public function on($event, $fn) {
        $this->hooks[$event][] = $fn;
    }

}

To call it statically:

Hook::on("click", function() {});

To call it dynamically:

$hook = new Hook;
$hook->on("click", function() {});