example.php

<?php

        $strategist = new Strategist();

        $strategist->respect(function () use ($var) {
            return true === $var; 
        })->then(function () use ($whatever) {
            return $whatever->getTrue(); 
        });
        $strategist->respect(function () use ($var) {
            return false === $var;
        })->then(function () use ($whatever) {
            return $whatever->getFalse();
        });
        $strategist->otherwise(function () use ($whatever) {
            return $whatever->default();
        });

        $result = $strategist->resolve(); // depend of matched strategy

strategist.php

<?php

declare(strict_types=1);

namespace Strategy;

class Strategist
{
    /**
     * @var Strategy[]
     */
    private $strategies;

    /**
     * @var callable
     */
    private $otherwise;

    public function __construct()
    {
        $this->strategies = [];
    }

    /**
     * @param $spec
     *
     * @return Strategy
     */
    public function respect($spec) : Strategy
    {
        $strategy           = new Strategy($spec);
        $this->strategies[] = $strategy;

        return $strategy;
    }

    /**
     * @param callable $callable
     *
     * @return Strategist
     */
    public function otherwise(callable $callable) : self
    {
        $this->otherwise = $callable;

        return $this;
    }

    /**
     * @return mixed
     */
    public function resolve()
    {
        $result    = null;
        $otherwise = $this->otherwise;

        foreach ($this->strategies as $strategy) {
            if (false === $strategy->isStatisfied()) {
                continue;
            }

            $result = null;

            $stack = $strategy->getHandlerStack();

            $stack->rewind();

            while ($stack->valid()) {
                $handler = $stack->current();

                try {
                    $result = $handler($result);
                } catch (\Throwable $e) {
                    return $otherwise($e);
                }

                $stack->next();
            }

            $this->strategies = [];

            return $result;
        }

        return $otherwise();
    }
}

strategy.php

<?php

declare(strict_types=1);

namespace Strategy;

class Strategy
{
    /**
     * @var callable
     */
    private $spec;

    /**
     * @var callable[]
     */
    private $handlerStack;

    /**
     * Strategy constructor.
     *
     * @param callable $spec
     */
    public function __construct(callable $spec)
    {
        $this->spec         = $spec;
        $this->handlerStack = new \SplStack();
    }

    /**
     * @return bool
     */
    public function isStatisfied() : bool
    {
        return ($this->spec)();
    }

    /**
     * @param callable $callable
     *
     * @return Strategy
     */
    public function then(callable $callable) : self
    {
        $this->handlerStack->push($callable);

        return $this;
    }

    /**
     * @return \SplStack
     */
    public function getHandlerStack() : \SplStack
    {
        return $this->handlerStack;
    }
}