TypeError: b.split is not a function error

Question

I'm trying to get data from MySQL via vue -> axios -> php. In response to the request, cath(error) fires with the following error:

TypeError: b.split is not a function
    at index.js:1:1
    at new Promise (<anonymous>)
    at t.exports (index.js:1:1)
    at t.exports (index.js:1:1)
    at h.request (index.js:1:1)
    at h.n.forEach.h.<computed> [as get] (index.js:1:1)
    at Function.get (index.js:1:1)
    at Vue.getDrivers (drivers.php:78:23)
    at Vue.mounted (drivers.php:95:18)
    at invokeWithErrorHandling (vue.js:1872:59)

in this file (drivers.php)

<script>
    var app = new Vue({
        el: '#newdriver',
        data: {
            drivers: [],
        },

        methods: {
            getDrivers() {
                axios.get({
                    url: 'scripts/drivers-action.php',
                    method: 'get',
                })
                    .then((res) => {
                        console.log("result");
                        console.log(res);
                        this.drivers = res.data.rows;
                    })
                    .catch((error) => {
                        // handle error
                        console.log(error);
                    });
            },
        },

        mounted: function() {
            this.getDrivers();
        },
    });
</script>

drivers-action.php:

<?php
    include('../dbcon.php');

    $sql = "SELECT * FROM drivers";
    $rows = getAllDrivers();

    function getAllDrivers() {
        $data = [];
        $statement = $conn->prepare($sql);
        if($statement->execute()) {
            $data = $statment->fetchAll();
        }
        return $data;
    }

    $data = array('rows' => $rows);
    echo json_encode($data);
?>

I can't figure out what is the problem and what is causing this error.

EDITED: dbconn.php, where i define $conn object for database connect

<?php
    $conn = new mysqli(
        'localhost',
        'root',
        '',
        'beldum'
    );
    if($conn -> connect_error) {
        die('Error: ('.$conn->connect_errno.')'.$conn->connect_error);
    }
?>