How to use core php function In Symfony framework Twig file

Firstly, create Twig/Extension directory into your bundle. Sample: AppBundle/Twig/Extension

Then, create a class for your function name. I create a JsonDecode and i use every twig file this function;

namespace AppBundle\Twig\Extension;

class JsonDecode extends \Twig_Extension {
    public function getName() {
        return 'twig.json_decode';
    }
    public function getFilters() {
        return array(
            'json_decode' => new \Twig_Filter_Method($this, 'jsonDecode')
        );
    }
    public function jsonDecode($string) {
        return json_decode($string);
    }
}

Then,

add lines into services.yml;

twig.json_decode:
    class: AppBundle\Twig\Extension\JsonDecode
    tags:
        - { twig.extension }

that's enough.