php calendar first day monday - php

Question

how can i change the first day in the first column of weeks into Monday.
here's the code i got from a website, and I've been trying to change the position of the days but still it displays the first day of the week as Sunday. how can i change it to Monday, thanks in advance
if(isset($_POST['func']) && !empty($_POST['func'])){
switch($_POST['func']){
case 'getCalender':
getCalender($_POST['year'],$_POST['month']);
break;
case 'getEvents':
getEvents($_POST['date']);
break;
default:
break;
}
}
/*
* Get calendar full HTML
*/
function getCalender($year = '',$month = '')
{
$dateYear = ($year != '')?$year:date("Y");
$dateMonth = ($month != '')?$month:date("m");
$date = $dateYear.'-'.$dateMonth.'-01';
$currentMonthFirstDay = date("N",strtotime($date));
$totalDaysOfMonth = cal_days_in_month(CAL_GREGORIAN,$dateMonth,$dateYear);
$totalDaysOfMonthDisplay = ($currentMonthFirstDay == 7)?($totalDaysOfMonth):($totalDaysOfMonth + $currentMonthFirstDay);
$boxDisplay = ($totalDaysOfMonthDisplay <= 35)?35:42;
?>
<div id="calender_section">
<h2>
<<
<select name="month_dropdown" class="month_dropdown dropdown"><?php echo getAllMonths($dateMonth); ?></select>
<select name="year_dropdown" class="year_dropdown dropdown"><?php echo getYearList($dateYear); ?></select>
>>
</h2>
<div id="event_list" class="none"></div>
<div id="calender_section_top">
<ul>
<li>Po</li>
<li>Út</li>
<li>St</li>
<li>Čt</li>
<li>Pá</li>
<li>So</li>
<li>Ne</li>
</ul>
</div>
<div id="calender_section_bot">
<ul>
<?php
$dayCount = 1;
for($cb=1;$cb<=$boxDisplay;$cb++){
if(($cb >= $currentMonthFirstDay+0 || $currentMonthFirstDay == 7) && $cb <= ($totalDaysOfMonthDisplay)){
//Current date
$currentDate = $dateYear.'-'.$dateMonth.'-'.$dayCount;
$eventNum = 0;
//Include db configuration file
include 'dbConfig.php';
//Get number of events based on the current date
$result = $db->query("SELECT title FROM events WHERE date = '".$currentDate."' AND status = 1");
$eventNum = $result->num_rows;
//Define date cell color
if(strtotime($currentDate) == strtotime(date("Y-m-d"))){
echo '<li date="'.$currentDate.'" class="grey date_cell">';
}elseif($eventNum > 0){
echo '<li date="'.$currentDate.'" class="light_sky date_cell">';
}else{
echo '<li date="'.$currentDate.'" class="date_cell">';
}
//Date cell
echo '<span>';
echo $dayCount;
echo '</span>';
//Hover event popup
echo '<div id="date_popup_'.$currentDate.'" class="date_popup_wrap none">';
echo '<div class="date_window">';
echo '<div class="popup_event">Events ('.$eventNum.')</div>';
echo ($eventNum > 0)?'view events':'';
echo '</div></div>';
echo '</li>';
$dayCount++;
?>
<?php }else{ ?>
<li><span> </span></li>
<?php } } ?>