php calendar คัยพอทราบวิธีทำให้ปฏิทินข้างล่างนี้คลิกย้อนหลังไม่ได้บ้างค่ะ

<?
  $mon=$_POST['mon'];
  $a = date(j); // , 1 to 31 
  $b = date(a); // am,
  $c = date(A); // AM,
  $d = date(B); // Swatch Internet time 000 -999
  $e = date(d); // , 01 to 31
  $f = date(D); // , sat 
  $g = date(F); // , November 
  $h = date(g); // , 1 through 12
  $hh = date(G); // , 0 through 23
  $i = date(h); // , 01 through 12 
  $j = date(H); // , 00 through 23 
  $k = date(i); // , 00 to 59 
  $l = date(I); // 0, 1 if Daylight Savings Time, 0 otherwise. 
  $m = date(l); // , Sunday through Saturday
  $n = date(L); // 0, 1 if it is a leap year, 0 otherwise.
  $o = date(m); // , 01 through 12
  $p = date(M); // , Jan through Dec
  $q = date(n); // , 1 through 12
  $r = date(o); // Example: +0200 
  $s = date(r); // Example: Thu, 21 Dec 2000 16:01:07 +0200
  $t = date(s); // 00 through 59
  $u = date(S); // st, nd, rd or th. Works well with j 
  $v = date(t); // 28 through 31
  $w = date(T); // time zoneseting Examples: EST, MDT ...
  $x = date(u); // See also time()
  $y = date(w); // 0 (for Sunday) through 6 (for Saturday)
  $z = date(W); // Example: 42 (the 42nd week in the year)
  $aa = date(y); // Examples: 99 or 03
  $bb = date(Y); // Examples: 1999 or 2003
  $cc = date(z); 
  $dd = date(Z); 
if(!$mon){
  $mon = $o ;
}else{
  $mon = $mon ;
}
if(!$year){
  $year = $bb ;
}else{
  $year = $year;
}
  $firstday = mktime(0,0,0,$mon,1,$bb);
  $firstdays = date('w',$firstday); 
  $lastdays = date('t',$firstday); 
  $year_view = date('Y',$firstday); 
  $mon_view = date('m',$firstday); 
echo"
<script>
function d_s( su , mnp){ 
if(mnp == 'm'){
document.dat_form.mon.value = su - 1 ;
}else{
document.dat_form.mon.value = su + 1 ;
}
document.dat_form.submit();
}
</script>
<form name=dat_form action='$PHP_SELF' method=post>
<input type=hidden name=mon value=''>
<table border=2 width=600 cellpadding=2 cellspacing=-2 bordercolor=#42C1FF>
<tr bgcolor=#FFCCCC>
<td colspan=7 align=center><a href=\"javascript:d_s($mon,'m')\"> Last Month.<< </a>&nbsp;$year_view - $mon_view <a href=\"javascript:d_s($mon,'p')\" > Next Month.>></a></td>
</tr>
</form>
<tr align=center><td><font Tahoma size=3 color=#FF0000>อาทิตย์</a></td><td><font Tahoma size=3color=#FFFF00>จันทร์</a></td><td><font Tahoma size=3 color=#FF66CC>อังคาร</a></td><td><font Tahoma size=3 color=#00CC00> &nbsp;&nbsp;พุธ&nbsp;&nbsp; </a></td><td><font Tahoma size=3 color=#FF6600>พฤหัสฯ</a></td><td><font Tahoma size=3 color=#00CCFF>ศุกร์</a></td><td><font Tahoma size=3 color=#6600CC>เสาร์</font></td></tr>
<tr align=right>";
//// 
for($i = 0; $i < 7 ; $i++){
if($firstdays == $i){
break;
}
else{
echo"<td width8=>&nbsp;</td>";
}
$week++; 
}
for($i=1; $i<=$lastdays ;$i++){
if($week % 7 == 0){
echo"</tr><tr align=right>";
}
if($a == $i){ 
$bgcolor = "bgcolor=#FFCCCC";
}else{
$bgcolor = 'bgcolor=white';
} 
if($week%7==0){ 
echo"<td $bgcolor><font Tahoma size=3 color=white><a href='Reservation_Select.php?day=$i&mon=$mon&year=$year'>$i</a></font></td>";
}else if($week%7==6){ 
echo"<td $bgcolor><font Tahoma size=3 color=white><a href='Reservation_Select.php?day=$i&mon=$mon&year=$year'>$i</a></font></td>";
}else{ // 
echo"<td $bgcolor><font Tahoma size=3><a href='Reservation_Select.php?day=$i&mon=$mon&year=$year'>$i</a></font></td>";
}
$week++;
}
while($week%7!=0){
echo"<td>&nbsp;</td>";
$week++;
}
echo"</tr>
</table>
";
?>