Why is Fatal error: Function name must be a string in returned?

Question

PHP

I'm trying to add middleware support to my project by adding a pipeline, but an error is returned:

Function name must be a string in /home/ugwwspov/bot.qeo.su/hardown/vk/VK/Pipeline/Pipeline.php on line 29

The code itself:

<?php

class Pipeline
{
    /**
     * @var callable[]
     */
    private $stages;

    public function pipe(callable $stage)
    {
        $this->stages[] = $stage;
        return $this;
    }

    public function __invoke($context, $next = null)
    {
        $this->start($context, $next);
    }

    public function start($context, $next = null)
    {
        if(!$current = array_shift($this->stages) and $next != null)
        {
            $next($context);
        }
        return $current($context, function ($context) use ($next) {
            return $this($context, $next);
        });
    }
}

Answer 1

Either you have a PHP server below version 5.3.0 (does not allow you to use the __invoke magic method).
Or try adding $this to the closure (where use is).

return $current($context, function ($context) use ($this, $next) {
            return $this($context, $next);