PHP/PDO: Insert data into foreign column - php

Question

i am very new at all of this... Here is my code (which i originally got from here)
<?php
/**
* Use an HTML form to create a new entry in the
* users table.
*
*/
if (isset($_POST['submit'])) {
require "../config.php";
require "../common.php";
try {
$connection = new PDO($dsn, $username, $password, $options);
$new_user = array(
"firstname" => $_POST['firstname'],
"lastname" => $_POST['lastname'],
"email" => $_POST['email'],
"age" => $_POST['age'],
"location" => $_POST['location']
);
$sql = sprintf(
"INSERT INTO %s (%s) values (%s)",
"users",
implode(", ", array_keys($new_user)),
":" . implode(", :", array_keys($new_user))
);
$statement = $connection->prepare($sql);
$statement->execute($new_user);
} catch(PDOException $error) {
echo $sql . "<br>" . $error->getMessage();
}
}
?>
<?php require "templates/header.php"; ?>
<?php if (isset($_POST['submit']) && $statement) { ?>
<blockquote><?php echo $_POST['firstname']; ?> successfully added.</blockquote>
<?php } ?>
<h2>Add a user</h2>
<form method="post">
<label for="firstname">First Name</label>
<input type="text" name="firstname" id="firstname">
<label for="lastname">Last Name</label>
<input type="text" name="lastname" id="lastname">
<label for="email">Email Address</label>
<input type="text" name="email" id="email">
<label for="age">Age</label>
<input type="text" name="age" id="age">
<label for="location">Location</label>
<input type="text" name="location" id="location">
<input type="submit" name="submit" value="Submit">
</form>
Back to home
<?php require "templates/footer.php"; ?>
This code worked just fine without fk columns. But in PHPAdmin I now made "location" a foreign column. The parent table looks like this:
Location
location INT (PK)
location name VARCHAR
I added a few locations to the parent table but still when i execute the code above, this error appears:
SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'location' cannot be null
How do I define the foreign key? Do I need a SELECT statement? I tried several things but I just cannot make it work. I'm about to give up so I'm posting on here.
Thank you very much for any answer!
PS: Maybe anybody can recommend any reading/books on this specific topic of PHP/PDO/SQL?