출처 :: https://www.phpschool.com/link/tipntech/65572

-> 일부 php 문법 수정함

<?php

/********** 사용자 설정값 **********/ 

$startYear = date( "Y" );

$endYear = date( "Y" ) + 30;

/********** 입력값 **********/ 

$year = ( $_GET['toYear'] )? $_GET['toYear'] : date( "Y" );

$month = ( $_GET['toMonth'] )? $_GET['toMonth'] : date( "m" );

$doms = array( "일", "월", "화", "수", "목", "금", "토" );

/********** 계산값 **********/ 

$mktime = mktime( 0, 0, 0, $month, 1, $year ); // 입력된 값으로 년-월-01을 만든다 

$days = date( "t", $mktime ); // 현재의 year와 month로 현재 달의 일수 구해오기 

$startDay = date( "w", $mktime ); // 시작요일 알아내기 

// 지난달 일수 구하기 

$prevDayCount = date( "t", mktime( 0, 0, 0, $month, 0, $year ) ) - $startDay + 1; 

$nowDayCount = 1; // 이번달 일자 카운팅 

$nextDayCount = 1; // 다음달 일자 카운팅 

// 이전, 다음 만들기 

$prevYear = ( $month == 1 )? ( $year - 1 ) : $year; 

$prevMonth = ( $month == 1 )? 12 : ( $month - 1 ); 

$nextYear = ( $month == 12 )? ( $year + 1 ) : $year; 

$nextMonth = ( $month == 12 )? 1 : ( $month + 1 ); 

// 출력행 계산 

$setRows = ceil( ( $startDay + $days ) / 7 ); 

?>

<center> 

<table style="border-collapse:collapse;"> 

    <form method="get"> 

    <tr> 

        <td style="padding:10;border-width:1;border-style:solid;"> 

        <select name="toYear" onchange="submit();"> 

        <?php for( $i = $startYear; $i < $endYear; $i++ ){ ?> 

        <option value="<?php echo $i; ?>" <?php echo ($i==$year)?"selected":""; ?>><?php echo $i; ?></option> 

        <?php } ?> 

        </select>년 

        <select name="toMonth" onchange="submit();"> 

        <?php for( $i = 1; $i <= 12; $i++ ) { ?> 

        <option value="<?php echo $i; ?>" <?php echo ($i==$month)?"selected":""; ?>><?php echo $i; ?></option> 

        <?php } ?> 

        </select>월 

        </td> 

    </tr> 

    </form> 

</table> 

<br> 

<table style="border-collapse:collapse;"> 

    <tr> 

        <td style="padding:10;border-width:1;border-style:solid;"> 

        <input type="button" onclick="location.href='<?php echo $_SERVER['PHP_SELF']; ?>?toYear=<?php echo $prevYear; ?>&toMonth=<?php echo $prevMonth; ?>'" value=" << "> 

        <?php echo $year; ?>년 <?php echo $month; ?>월 

        <input type="button" onclick="location.href='<?php echo $_SERVER['PHP_SELF']; ?>?toYear=<?php echo $nextYear; ?>&toMonth=<?php echo $nextMonth; ?>'" value=" >> "> 

        </td> 

    </tr> 

</table> 

<br> 

<table cellpadding=0 cellspacing=0 style="border-collapse:collapse;"> 

    <tr> 

        <?php for( $i = 0; $i < count( $doms ); $i++ ) { ?> 

        <td align="center" style="padding:6;border-width:1;border-style:solid;"><?php echo $doms[$i]; ?>요일</td> 

        <?php } ?> 

    </tr> 

    <?php for( $rows = 0; $rows < $setRows; $rows++ ) { ?> 

    <tr> 

        <?php 

        for( $cols = 0; $cols < 7; $cols++ ) 

        { 

            // 셀 인덱스 만들자 

            $cellIndex    = ( 7 * $rows ) + $cols; 

            ?> 

            <?php 

            // 이번달이라면 

            if ( $startDay <= $cellIndex && $nowDayCount <= $days ) { ?> 

            <td align="center" style="padding:6;border-width:1;border-style:solid;"> 

                <?php if ( date( "w", mktime( 0, 0, 0, $month, $nowDayCount, $year ) ) == 6 ) { ?> 

                <b><font color="blue"><?php echo $nowDayCount++; ?></font></b> 

                

                <?php } else if ( date( "w", mktime( 0, 0, 0, $month, $nowDayCount, $year ) ) == 0 ) { ?> 

                <b><font color="red"><?php echo $nowDayCount++; ?></font></b> 

                

                <?php } else { ?> 

                <b><?php echo $nowDayCount++; ?></b> 

                <?php } ?> 

            </td> 

            

            <?php 

            // 이전달이라면 

            } else if ( $cellIndex < $startDay ) { ?> 

            <td align="center" style="padding:6;border-width:1;border-style:solid;"> 

            <font color="gray"><b><?php echo $prevDayCount++; ?></b></font> 

            </td> 

            

            <?php 

            // 다음달 이라면 

            } else if ( $cellIndex >= $days ) { ?> 

            <td align="center" style="padding:6;border-width:1;border-style:solid;"> 

            <font color="gray"><b><?php echo $nextDayCount++; ?></b></font> 

            </td> 

            <?php } 

        } 

        ?> 

    </tr> 

    <?php } ?> 

</table>

</center>