print_r($monthName);

<script>
$(function () {
var monthNameData = [];
    monthNameData.push({
        data: ['<?php echo join ($monthName, ',') ?>']
    });

    $('#iw').highcharts({
        chart: {
            ...
        xAxis: {
           categories: monthNameData[0].data
        },
        series: ...
    });
});