how to insert selected options into a database - php

Question

I am building a php based quiz for website. I am a beginner. I coded for getting the questions and options from database and display them in the form in my page.when the user selects the option i want the the answers to be inserted to my database.I wrote some bsic code but its not working pls help. Here is my code
The table for question is having 7rows(qid,question,optiona,optionb,optionc,optiond,answeroption),answers table is having simple2rows(qid,answer)
if(isset($_POST['next']))
{
$a=$_POST['a'];
}
if(!isset($a))
{
$a=0;
}
include('connection.php');
mysql_query("INSERT INTO answers (username,qid, option)
VALUES ($username,a-1,'$_POST('option'))");
$sql1="SELECT * FROM exam1 LIMIT 1 OFFSET $a";
$result=mysql_query($sql1);
echo "<form method='post' action='quiz.php'>";
while ($row = mysql_fetch_array($result))
{
echo $row['question']. "<br/>";
echo "<input type='radio' value='optiona' name='option'>" .$row['optiona'];
echo "<input type='radio' value='optionb' name='option'>" .$row['optionb'];
echo "<input type='radio' value='optionc' name='option'>" .$row['optionc'];
echo "<input type='radio' value='optiond' name='option'>" .$row['optiond']; "<br/>";
}
$c=$a-1;
$b=$a+1;
echo "<input type='hidden' value='$c' name='a'>";
echo "<input type='submit' name='previous' value='previous'> ";
echo "<input type='hidden' value='$b' name='a'>";
echo "<input type='submit' name='next' value='next'> ";
echo "<input type='reset' name='reset' value='Reset'>";
echo "</form>";
?>

Answer 1

You need to study up on basic PHP syntax. $_POST is not a function. It's an array. e.g.
$var = $_POST['var'];
^-- ^--- note the bracketing.
Even if the code DID work, you'd be wide open to SQL injection attacks.

Answer 2

Big big atention! I hope this is just an test code note final one!
Your code is Inject Vulnerably here $a=$_POST['a'];
I recomend to replace with this one $a= filter_input(INPUT_POST, 'a', FILTER_SANITIZE_STRING); this is an escaped from attacks!
I hope it hepls you