Ajax calendar class

<?php

/*********************/
//This is an example with all possible calendar customizations and 
//generating calendar and ajax request in the same file
/*********************/
//All methods are divided in 2 groups
//One should be used in same file or scope, where calendar will be generated
//Other should be used in same file or scope, where ajax request will be made
//The ones used in if(isset($_GET['ajax_calendar'])) clause 
//should be used within request file or scope
//The ones in else clause should be used within calendar generator file or scope

//you should always sepecify timezone when dealing with date function like used in calendar
date_default_timezone_set("Europe/Helsinki");

//declaring class instance
include("./calendar.class.php");
$calendar = new calendar();


if(isset($_GET['ajax_calendar']))
{
//if $_GET['ajax_calendar'] is set, it means request is beeing made 
//by ajax to get days  of other months or year
//make sure that you do not output anything to browser, 
//so you won't mess json structure of calendar
//other parameters that are passed with this request are
//$_GET['date'] - for date
//$_GET['month'] - for month
//$_GET['year'] - for year
//$_GET['start'] - which day to start week with
//and any other custom parameters added
//for this example it is parameter $_GET['param'] = "value"

    //set start month and year which is an oldest motnh to show
    $calendar->set_start_date(1,1987);
    //set end month and year which is a newest motnh to show
    $calendar->set_end_date(3,1987);
    
    //set url pattern, for all active days to be used as links 
    where $1 will be replaced by date, $2 replaces by month and $3 by year, 
    $calendar->set_url_pattern("javascript:alert('Date selected $3-$2-$1');");
    //$arr = array(1 => "#beginning_of_a_month", 30 => "#end_of_a_month");
    //$calendar->set_events($arr);
    
    //if ajax request is made to the same page, 
    //make sure nothing other is outputted to browser
    //you can specify request url in the file where you generate calendar
    //if nothing is specified then request will be made to the same page
    $calendar->process_request();
}
else
{
    //defining path to jquery and style css included in package
    echo '<script type="text/javascript" src="./jquery.js"></script>';
    echo '<style type="text/css" media="all">@import "./style.css";</style>';
    //array with custom month names
    $months = array("Jan", "Feb", "Mar", "Apr", "May", "Jun", 
                    "Jul", "Aug", "Sep", "Oct", "Nov", "Dec");
    //applying custom month names
    $calendar->set_months($months);
    
    //array with custom days of week names
    $weekdays = array("M", "T", "W", "Th", "F", "S", "Sun");
    //applying custom days of week names
    $calendar->set_weekdays($weekdays);
    
    //define with which day to start week 0 - Monday, ..., 6 - Sunday
    //here in example it is Tuesday
    $calendar->set_week_start(1);
    
    //adding request parameter name and value which will appear in ajax request
    $calendar->add_request_param("param", "value");
    
    //getting errors
    $errors = $calendar->get_errors();
    if(!empty($errors))
    {
        foreach($errors as $error)
        {
            echo "<p>".$error."</p>";
        }
    }
    
    //generating calendar, you can specify date like in example
    //if no date is specified, than todays date will be used
    $calendar->generate_calendar(23,2,1987);
}
?>

/*table for calendar header with month, year etc*/
.ajax_calendar_header
{
    border:0;
    width:250px;
}
/*table cell that contains month and year*/
.ajax_calendar_main_date
{
    font-weight: bold;
    width: 60%;
    text-align: center;
}
/*table cell of link to switch to previous month when active 
(means can be switched to previuos months)*/
.ajax_calendar_prev_month_active 
{
    width: 10%;
    font-weight: bold;
}
/*customizing link inside of active cell (inactive cells doesn't have links)*/
.ajax_calendar_prev_month_active a
{
    text-decoration: none;
}
/*table cell of link to switch to previous month when inactive 
(means can not be switched to previous months because of start_date restriction)*/
.ajax_calendar_prev_month_inactive
{
    width: 10%;
    color: gray;
}
/*table cell of link to switch to previous year when active*/
.ajax_calendar_prev_year_active
{
    width: 10%;
    font-weight: bold;
}
/*customizing link inside of active cell (inactive cells doesn't have links)*/
.ajax_calendar_prev_year_active a
{
    text-decoration: none;
}
/*table cell of link to switch to previous year when inactive*/
.ajax_calendar_prev_year_inactive
{
    width: 10%;
    color: gray;
}
/*table cell of link to switch to next month when active*/
.ajax_calendar_next_month_active
{
    width: 10%;
    font-weight: bold;
}
/*customizing link inside of active cell (inactive cells doesn't have links)*/
.ajax_calendar_next_month_active a
{
    text-decoration: none;
}
/*table cell of link to switch to next month when inactive*/
.ajax_calendar_next_month_inactive
{
    width: 10%;
    color: gray;
}
/*table cell of link to switch to next year when active*/
.ajax_calendar_next_year_active
{
    width: 10%;
    font-weight: bold;
}
/*customizing link inside of active cell (inactive cells doesn't have links)*/
.ajax_calendar_next_year_active a
{
    text-decoration: none;
}
/*table cell of link to switch to next year when inactive*/
.ajax_calendar_next_year_inactive
{
    width: 10%;
    color: gray;
}
/*table with calendar itself*/
.ajax_calendar
{
    border:6px outset black;
    width:250px;
}
/*cells of calendar*/
.ajax_calendar td
{
    border: 1px solid black;
    text-align: center;
}
/*table row with days of the week*/
.ajax_calendar_weekdays
{
    font-weight: bold;
}
/*table cells with inactive days from previous or next months*/
.ajax_calendar_inactive_days
{
    color:black; 
    background-color: gray;
}
/*table cells with active days from currently selected month*/
.ajax_calendar_active_days
{
    color: blue;
}
/*table cell with currently selected date*/
.ajax_calendar_today_days
{
    color: red;
    background-color: yellow;
}