//index.php
if(isset($_POST['btn-signup-preduzece'])) {
    $uname = trim($_POST['pr-username']); //there are inputs in my html
    $umail = trim($_POST['pr-email']);
    $upass = trim($_POST['pr-password']);
    $comp = trim($_POST['pr-naziv']);
    $maticni = trim($_POST['pr-maticni']);
    $pib = trim($_POST['pr-pib']);
    $sifra = trim($_POST['pr-sifra']);
    $racun = trim($_POST['pr-racun']);
    $adresa = trim($_POST['pr-adresa']);

    if($uname=="") {
        $error[] = "provide username !";
    }
    else if($umail=="") {
        $error[] = "provide email id !";
    }
    else if(!filter_var($umail, FILTER_VALIDATE_EMAIL)) {
        $error[] = 'Please enter a valid email address !';
    }
    else if($upass=="") {
        $error[] = "provide password !";
    }
    else {
        try {
            $stmt = $DB_con->prepare("SELECT username,email FROM preduzeca WHERE username=:uname OR email=:umail");
            $stmt->execute(array(':uname'=>$uname, ':umail'=>$umail));
            $row=$stmt->fetch(PDO::FETCH_ASSOC);

            if($row['username']==$uname) {
                $error[] = "sorry username already taken !";
            }
            else if($row['email']==$umail) {
                $error[] = "sorry email id already taken !";
            }
            else {
                //PROBLEM IS HERE IN THIS FUNTION BELLOW, when I put here some echo it writes me that, but if i put echo bellow this if statement it gives me nothing
                if($user->registerPreduzece($uname,$upass,$umail, $comp, $maticni, $pib, $sifra, $racun, $adresa)) {
                    $user->redirect('ostalo/uspesno.php');
                }
            }
        }
        catch(PDOException $e) {
            echo $e->getMessage();
        }
    }
}

public function registerPreduzece($uname,$upass,$umail, $comp, $maticni, $pib, $sifra, $racun, $adresa) {
   try
   {
       $new_password = password_hash($upass, PASSWORD_DEFAULT);

       $stmt = $this->db->prepare("INSERT INTO preduzeca(naziv,maticniBroj,PIB,sifraDelatnosti,racun,adresa,username,password,email)
                                                   VALUES(:comp, :maticni, :pib, :sifra, :racun, :adresa, :uname, :upass, :umail)");
       $stmt->bindparam(":naziv", $comp);
       $stmt->bindparam(":maticniBroj", $maticni);
       $stmt->bindparam(":PIB", $pib);
       $stmt->bindparam(":sifraDelatnosti", $sifra);
       $stmt->bindparam(":racun", $racun);
       $stmt->bindparam(":adresa", $adresa);
       $stmt->bindparam(":username", $uname);
       $stmt->bindparam(":password", $new_password);
       $stmt->bindparam(":email", $umail);
       $stmt->execute();

       return $stmt;
   }
   catch(PDOException $e)
   {
       echo $e->getMessage();
   }
}

CREATE TABLE `preduzeca` (
`idPreduzeca` int(10) UNSIGNED NOT NULL,
`naziv` varchar(45) NOT NULL,
`maticniBroj` varchar(8) DEFAULT NULL,
`PIB` varchar(11) DEFAULT NULL,
`sifraDelatnosti` varchar(5) DEFAULT NULL,
`racun` varchar(20) DEFAULT NULL,
`adresa` int(11) DEFAULT NULL,
`username` varchar(45) NOT NULL,
`password` varchar(45) NOT NULL,
`email` varchar(45) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

ALTER TABLE `preduzeca`
ADD PRIMARY KEY (`idPreduzeca`),
ADD UNIQUE KEY `idfirme_UNIQUE` (`idPreduzeca`),
ADD UNIQUE KEY `username_UNIQUE` (`username`),
ADD UNIQUE KEY `maticniBroj_UNIQUE` (`maticniBroj`),
ADD UNIQUE KEY `PIB_UNIQUE` (`PIB`),
ADD UNIQUE KEY `racun_UNIQUE` (`racun`),
ADD KEY `fk_preduzeca_adrese1_idx` (`adresa`);