how to validate a form in php?

[impin]

how to validate this form. when user submits without selecting the options, must get a alert.

my code

echo "<form method='post' id='submit' action='checkresult.php'>";
$sql="SELECT * FROM cquestions where showdate='$today' limit 2";
$result=mysql_query($sql);
while ($row = mysql_fetch_array($result)) {
echo "<p>" . $row['cqtext'] . "</p>";
$sql2="SELECT * FROM canswers where cqid=".$row['cqid'];
$result2=mysql_query($sql2);
while($row2=mysql_fetch_assoc($result2))
{
echo "<input type='radio' name='".$row['cqid']."' value='".$row2['cqans']."' />".$row2['aatext']; }
}

[Rabbit]

In the page that it is submitted to, check that the fields were filled out and echo an error message if it wasn't complete.

[Januardi1793]

You can use html, put "required" into tag input

like this :

echo "<input type='radio' name='".$row['cqid']."' value='".$row2['cqans']."' required />".$row2['aatext'];