Undefined function even tho it is defined

So this is my entire test :

<?php 

class FirstCest
{

    public function __rand_float($st_num=0,$end_num=1,$mul=1000000)
    {
        if ($st_num>$end_num) return false;
        $this->__rand_float();
        return mt_rand($st_num*$mul,$end_num*$mul)/$mul;
    }



    public function _before(AcceptanceTester $I)
    {

        //Times to answer questions

        $nq = 8;        //number of questions
        $ncand = 5;     //number of candidates
        $lexam = 10;    //minutes
        $smpu = 0.05;
        $lexam1 = $lexam * (1- $smpu);

        $times = [];

        //for ($i=$ncand; $i > 0; $i--) { 

            $lexam2 = $lexam1 * ( 0.9 + __rand_float(0,0.2,25));
            $tques = round($lexam2 / $nq,3);
            $ta = 0;
                for ($i=$nq; $i > 0 ; $i--) { 
                    $tques1 = $tques * round((__rand_float(0.5,1.5,5)),2);
                    $ta = $ta + $tques1;
                    array_push($times, $tquest1);
                }
        //    }

                return $times;

    }

    // test
    public function loginSuccessfully(AcceptanceTester $I,$times)
    { 



        $trainees = ['008C108:8706405','008C408:1776723','008B108:9315051','008BD08:4357300','008BF08:4235645','008C308:8571541','008B008:7139762','008BB08:8942468','008BC08:6648592','6648592:8504197','008C608:6219682','008AE08:2728576','008AF08:9407850'];


        $user = $trainees[rand(0,count($trainees))];

        $credentials = explode(':',$user,2);

        unset($trainees[$user]);


        $I->amOnPage('');

        $I->waitForElementClickable('//*[@id="submitLogin"]');
        $I->fillField('username',$credentials[0]);
        $I->fillField('password',$credentials[1]);
        $I->click('//*[@id="submitLogin"]');

        //preliminaries
        $I->waitForElementClickable('//*[@id="Proceed"]');
        $I->click('//*[@id="Proceed"]');

        $I->waitForElementClickable('//*[@id="pCheckbox"]/span');
        $I->click('//*[@id="pCheckbox"]/span');

        $I->waitForElementClickable('//*[@id="Proceed"]');
        $I->click('//*[@id="Proceed"]');

        $I->waitForElementClickable('//*[@id="Proceed"]');
        $I->click('//*[@id="Proceed"]');

        $I->waitForElementClickable('//*[@id="pCheckbox"]/span');
        $I->click('//*[@id="pCheckbox"]/span');

        $I->waitForElementClickable('//*[@id="Proceed"]');
        $I->click('//*[@id="Proceed"]');

        $I->waitForElementClickable('//*[@id="Proceed"]');
        $I->click('//*[@id="Proceed"]');


        $I->waitForElementClickable('//*[@id="pCheckbox"]/span');
        $I->click('//*[@id="pCheckbox"]/span');

        $I->waitForElementClickable('//*[@id="Proceed"]');
        $I->click('//*[@id="Proceed"]');
        //start exam
        $I->waitForElementNotVisible('//*[@id="oleWaitMessage"]', 200);
        for ($q=count($times); $q > 0 ; $q--) { 
            $I->waitForElementClickable('//*[@id="rb_0"]');
            $I->wait($times[$q]);
            $I->click('//*[@id="rb_'.rand(0,2).'"]');
            $I->waitForElementClickable('//*[@id="oleNextItemBtn"]');
            $I->click('//*[@id="oleNextItemBtn"]');
        }
        $I->waitForElementClickable('//*[@id="oleSubmitBtn"]');
        $I->wait(2);
        $I->click('//*[@id="oleSubmitBtn"]');

        $I->waitForElementClickable('//*[@id="yui-gen0-button"]');
        $I->wait(2);
        $I->click('//*[@id="yui-gen0-button"]');
    }
}

As you can see I have a function called __rand_float() which I use in the _before method. So when I run this test I get the following:

There was 1 error:

---------
1) FirstCest: Login successfully
 Test  tests\acceptance\FirstCest.php:loginSuccessfully

  [Error] Call to undefined function __rand_float()

#1  D:\Programas\xampp\htdocs\robo2\tests\acceptance\FirstCest.php:30
#2  FirstCest->_before

What may be causing this?