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<?php

class AdWizard
{
    protected $steps = array();

    // ...

    /**
     * How to find a step with an identifier provided using array_filter function
     */
    public function step($identifier)
    {
        list($step) = \array_filter($this->steps, function($step) use ($identifier) {
            return $step->getIdentifier() == $identifier;
        });

        if (false == $step)) {
            throw new \InvalidArgumentException('The step with identifier `'.$identifier.'` was not found');
        }

        return $step;
    }
}