PHP mysql Online exam system

A simple PHP tutorial MySQL tutorial exam system, the example method is to determine whether the topic array is empty, empty then from the question bank randomly removed 12 questions ID

<?php
Include './inc/header.php ';
if ($_session[' arr '] = = ') {#判断题目数组是否为空, empty then randomly take out 12 questions ID from the question bank
$sql = ' select * from ';
$select = $db->select ($sql);
for ($i =0; $i <count ($select); $i + +) {
$aid. = $select [$i] [' ID ']. ', ';
}
$arr = Explode (', ', substr ($aid, 0,strlen ($aid)-1));
Shuffle ($arr);
$_session[' arr ' = Array_slice ($arr, 0,12);
}
$array = $_session[' arr '];
if ($_server[' request_method '] = = "Post") {
if ($_session[' arr1 '] = = "") {#arr1存的是已答题的id
$ary = $array;
Array_shift ($ary); #每答完一道题后, delete the first bit of the array
$_session[' arr1 ' = $ary;
}else{
$ary = $_session[' arr1 '];
Array_shift ($ary);
$_session[' arr1 ' = $ary;
}
$id = $_post[' id '];
$answer = $_post[' answer '];
$SQQ = ' select * from ' where id= '. $id. ' answer= '. $answer. ' "
$qus = mysql_query ($SQQ);
$rows = @mysql_num_rows ($qus);
if ($rows) {
$_session[' right ']. = $id. ', '; #将答对题id存到session [' Right ']
$rightarr = Explode (', ', substr ($_session[' right '],0,strlen ($_session[' right '))-1); #判断对题数
if (count ($rightarr) <) {#如果小于10则继续答下一题
$sql 2 = ' select * from where id= '. $ary [0];
$sel = $db->select ($sql 2);
}elseif (count ($rightarr) = =) {#如果等于10则跳出此次问答
$time = time ();
$_session[' jiang ' = MD5 ($time);
#echo ' <script>location.href= ' right.php?uid= '. $time. ' "; </script> ';
$success = Array (' Second ' => ' 3 ', ' url ' => ' right.php?uid= '. $time, ' title ' => ' answer ten questions correctly ', ' right ' => ' Congratulations, you've answered 10 straight questions. , you will jump to the redemption page. ');
$smarty->assign (' msg ', $success);
$smarty->display (' success.html ');
Exit ();
}
}else{
$_session[' ERROR ']. = $id. ', '; #将答错题id存到session [' ERROR ']
$errorarr = Explode (', ', substr ($_session[' Error '],0,strlen ($_session[' ERROR ')-1)); #判断错题数
if (count ($errorarr) < 2) {
$sql 2 = ' select * from where id= '. $ary [0];
$sel = $db->select ($sql 2);
}elseif (count ($errorarr) = = 2) {#如果答错两题, then count again
unset ($_session[' right ']);
unset ($_session[' error ')];
unset ($_session[' arr1 '));
unset ($_session[' arr '));
unset ($_session[' num ']);
Session_destroy ();
#echo ' <script>location.href= ' index.php ';</script> ';
$success = Array (' Second ' => ' 3 ', ' url ' => ' index.php ', ' title ' => ' wrong two questions ', ' error ' => ' Sorry, you have two consecutive wrong answers, will be recalculated ');
$smarty->assign (' msg ', $success);
$smarty->display (' success.html ');
Exit ();
}
}
Echo ' right: '. $_session[' right '. ' <br/> ';
Echo ' ERROR: '. $_session[' error '. ' <br/> ';
}else{
$SQLL = ' select * from where id= '. $array [0];
$sel = $db->select ($SQLL);
}
if (isset ($_session[' num ')) {#目前答第几题
$num = $_session[' num '] + 1;
$_session[' num '] = $num;
#echo $_session[' num ']. ' <br/> ';
#echo $num;
}else{
$num = 1;
$_session[' num '] = $num;
}
$smarty->assign (' num ', $num);
$smarty->assign (' ob ', $sel);
$smarty->display (' index.html ');