A classe UnderflowException

The most typical usage is with stack, queue or collection, for example when you queue tasks, make call stack or manipulate JSON, XML, etc. elements.As other exepctions of RuntimeException class, this type of error can't be detected in (for example) your IDE or compiler.<?php// Functions declared above $f1 = function() { setTypeControl('username');};$f2 = function() { setTypeControl('userpass');};$f3 = function() { setButton('Add');};$f4 = function() { setButton('OK');};$tasks = new class {    private $list;    // Create internal queue    public function __construct() {        $this->list = new SplQueue;    }    // Add to queue    public function add(callable $func) {        $this->list->enqueue($func);    }    // Delete from queue and execute    public function do() {        if ($this->list->isEmpty()) {            throw new UnderflowException;        } else {            call_user_func($this->list->dequeue());        }    }};$tasks->add($f1);$tasks->add($f2);$tasks->add($f3);$tasks->add($f4);$tasks->do(); // Field username is created$tasks->do(); // Field userpass is created$tasks->do(); // Button Add is created$tasks->do(); // Button OK is created$tasks->do(); // Fatal error: Uncaught UnderflowException in ...