MySql Entry Shows NULL

When you tick the optional checkbox to subscribe to my newsletter on my html form and when you submit the form, my Mysql Database entry "newsletter_subscribe_yes" column shows NULL entry. Why ?

Particular Form Input

<label for="newsletter_subcribe_yes">Newsletter Subcribe:Yes</label>
<input type="checkbox" name="newsletter_subcribe_yes" id="newsletter_subcribe_yes" value="Newsletter Subcribe:Yes">
<br>

Full Form Submission Script

<?php
ini_set('display_errors','1');
ini_set('display_startup_errors','1');
error_reporting(E_ALL);
?>

<form method="POST" name="textfield" id="textfield" action="" required>
<label for="domain">Domain:</label><br>
<input type="text" name="domain" id="website_domain" maxlength="255" size="25">
<br>
<label for="email">Email:</label><br>
<input type="email" name="domain_email" id="email_address" maxlength="255" size="20">
<br>
<label for="url">Url:</label><br>
<input type="url" name="url" id="website_address" maxlength="255" size="50">
<br>
<label for="text">Title:</label><br>
<input type="text" name="title" id="title" value="" maxlength="255" size="60">
<br>
<label for="text">Anchor:</label><br>
<input type="text" name="anchor" id="link_text" value="" maxlength="255" size="60">
<br>
<label for="description">Description:</label><br>
<textarea name="description" id="website_description" cols="50" rows="10"></textarea>
<br>
<label for="keywords">Keywords:</label><br>
<textarea name="keywords" id="website_keywords" cols="50" rows="10"></textarea>
<br>
<b>Terms & Conditions:</b>
<input type="radio" name="tos_agree" id="yes" value="Agree to TOS: Yes:"><label for="yes">Agree to TOS: Yes:</label>
<input type="radio" name="tos_agree" id="no" value="Agree to TOS: No:"><label for="no">Agree to TOS: No:</label>
<br>
<label for="newsletter_subcribe_yes">Newsletter Subcribe:Yes</label>
<input type="checkbox" name="newsletter_subcribe_yes" id="newsletter_subcribe_yes" value="Newsletter Subcribe:Yes">
<br>
<button type="reset">Reset!</button>
<br>
<button type="submit" name="submit" id="submit">Submit!</button><br>
</form>

<?php

$labels_assoc = array("domain_email"=>"required","domain"=>"required","url"=>"required","title"=>"not required","anchor"=>"required","description"=>"required","keywords"=>"required","tos_agree"=>"required","newsletter_subscribe_yes"=>"not required");
$labels_keys = array_keys($labels_assoc);

if($_SERVER['REQUEST_METHOD']==='POST')
{
    $errors = array();
    //Form Labels (name="") & MySql Table Column Names.
    $labels_assoc = array("domain_email"=>"required","domain"=>"required","url"=>"required","title"=>"not required","anchor"=>"required","description"=>"required","keywords"=>"required","tos_agree"=>"required","newsletter_subscribe_yes"=>"not required");
    $labels_keys = array_keys($labels_assoc);

    foreach($labels_assoc as $key=>$value)
    {
        if($value=="required")
        {
            if(empty($_POST[$key])||!is_string($_POST[$key]))
            {
                $errors[] = '<b>' .ucfirst($key) .'</b>'  .' is required!';
            }
            else
            {
                //Required Input
                if($key == 'domain_email' && !filter_input(INPUT_POST,"domain_email",FILTER_VALIDATE_EMAIL))
                {
                    die('Enter a valid Email!');
                }
                //Required Input
                elseif($key == 'domain' && !filter_input(INPUT_POST,"domain",FILTER_VALIDATE_DOMAIN))
                {
                    die('Enter a valid Domain!');
                }
                //Required Input
                elseif($key == 'url' && !filter_input(INPUT_POST,"url",FILTER_VALIDATE_URL))
                {
                    die('Enter a valid Url!');
                }
                //Optional Input
                elseif($key == 'title' && !EMPTY($key['title']) && !filter_input(INPUT_POST,"title",FILTER_SANITIZE_STRING))
                {
                    die('Enter a valid Page Title!');
                }
                elseif($key == 'anchor' && !filter_input(INPUT_POST,"anchor",FILTER_SANITIZE_STRING))
                {
                    die('Enter a valid Page Anchor!');
                }
                elseif($key == 'description' && !filter_input(INPUT_POST,"description",FILTER_SANITIZE_STRING))
                {
                    die('Enter a valid Page Description!');
                }
                elseif($key == 'tos_agree' && EMPTY($_POST['tos_agree']))
                {
                    die('You must agree to tos (terms & conditions)!');
                }
            }
        }
    }

    if(!empty($errors))
    {
        foreach($errors as $error)
        {
            echo '<b>Line ' .__LINE__ .': </b>'; echo 'Error Alert: ' .$error; echo '<br>';
        }
    }
    
    //TEST RESULT:
    //$_POST[''] outputs the value of value="".
    //For php to extract user input, the radio html needs to have the value="". Php cannot extract without it.


    //MYSQLI CONNECTION.
    mysqli_report(MYSQLI_REPORT_ERROR|MYSQLI_REPORT_STRICT);

    $server = 'localhost';
    $user = 'root';
    $password = '';
    $database = 'index';

    if(!$conn = mysqli_connect("$server","$user","$password","$database"))
    {
        echo 'Mysqli Connection Error' .mysqli_connect_error($conn);
        echo 'Mysqli Connection Error Number' .mysqli_connect_errno($conn);
    }

    mysqli_set_charset($conn,'utf8mb4');


    $col1 = $labels_keys["0"];
    $col2 = $labels_keys["1"];
    $col3 = $labels_keys["2"];
    $col4 = $labels_keys["3"];
    $col5 = $labels_keys["4"];
    $col6 = $labels_keys["5"];
    $col7 = $labels_keys["6"];
    $col8 = $labels_keys["7"];
    $col9 = $labels_keys["8"];

    $sql = "INSERT into links ($col1,$col2,$col3,$col4,$col5,$col6,$col7,$col8,$col9) VALUES (?,?,?,?,?,?,?,?,?)";
    $stmt = mysqli_stmt_init($conn);
    mysqli_stmt_prepare($stmt,$sql);
    mysqli_stmt_bind_param($stmt,'sssssssss',$_POST["$col1"],$_POST["$col2"],$_POST["$col3"],$_POST["$col4"],$_POST["$col5"],$_POST["$col6"],$_POST["$col7"],$_POST["$col8"],$_POST["$col9"]);
    mysqli_stmt_execute($stmt);
    mysqli_stmt_close($stmt);
    mysqli_close($conn);
    }

?>

I'd appreciate any mentions of any other coding errors on my script apart from the problem I mentioned.

Thank You!