<?php

namespace callable;

class format
    {

    static public function format1($value)
        {
        return strrev($value);
        }

    public function format2($value)
        {
        return $value . $value;
        }

    }

$obj = new format();

function format3($value)
    {
    return strtoupper($value);
    }

$format4 = function ($value)
        {
        return implode('SSSS', explode('S', $value));
        };
//------------------------------------------------------------------------------
$value = 'SoMeStRiNg';
$format1 = array('\callable\format', 'format1');
echo call_user_func($format1, $value) . PHP_EOL; //gNiRtSeMoS
$format2 = array($obj, 'format2');
echo call_user_func($format2, $value) . PHP_EOL; //SoMeStRiNgSoMeStRiNg
$format3 = '\callable\format3';
echo call_user_func($format3, $value) . PHP_EOL; //SOMESTRING
echo call_user_func($format4, $value) . PHP_EOL; //SSSSoMeSSSStRiNg
echo $format4($value) . PHP_EOL; //SSSSoMeSSSStRiNg