在多个表中添加记录PHP MYSQL

I have two tables, users and comments. I need to add firstname and lastname to users and the comments to the comments table with the user id as a foreign key in comments. I am using phpmyadmin to do the add the foreign key constrain and relationships.

This is my html form:

<form action="dbconnect.php" method="post">
Firstname: <input type="text" name="firstname"><br />
Lastname: <input type="text" name="lastname"><br />
Comments: <textarea cols="30" rows="5" name="comments"></textarea><br />
<input type="submit">
</form>

This is my php insert code:

mysql_select_db("test", $db_server);

$sql="INSERT INTO users (Firstname, Lastname)
VALUES
('$_POST[firstname]','$_POST[lastname]')";

$sql="INSERT INTO comments (Comment)
VALUES
('$_POST[comments]')";


if (!mysql_query($sql,$db_server))
    {
    die('Error' . mysql_error());   
    }
echo "1 record added" ;

mysql_close($db_server);

and this is the error i'm getting:

Error: Cannot add or update a child row: a foreign key constraint fails (test.comments, CONSTRAINT comments_ibfk_2 FOREIGN KEY (useridfk) REFERENCES users (id) ON DELETE CASCADE ON UPDATE CASCADE)

I am new to php and phpmyadmin so any help is appreciated.

First of all, you need to run the INSERT query on the users table so the user record is created.

mysql_select_db("test", $db_server);

$sql="INSERT INTO users (Firstname, Lastname)
VALUES
('$_POST[firstname]','$_POST[lastname]')";

mysql_query($sql);

if (!mysql_query($sql,$db_server))
    {
    die('Error' . mysql_error());   
    }
echo "1 record added" ;

You then need to retrieve the id of the newly created record on the users table

$newIdQuery = mysql_query("SELECT LAST_INSERT_ID() AS userId FROM users");
$newId = mysql_fetch_assoc($newIdQuery);

Then you can use that userId to add the comments, with the user id foriegn key properly populated.

Note: When building MySQL queries using the mysql_* functions, if you wrap a php variable in { } it will insert the value of the php variable into the SQL statement.

$sql="INSERT INTO comments (id, Comment)
VALUES
({$newId["userId"]}, '$_POST[comments]')";


if (!mysql_query($sql,$db_server))
    {
    die('Error' . mysql_error());   
    }
echo "1 record added" ;

mysql_close($db_server);