Button form submission not submitting NULL - php

Question

I am working with a fairly straight forward form yet for some reason it is not submitting NULL results and instead submits them as empty results.
<div class="span6">
<form action="" method="POST">
<div class="block-fluid without-head">
<div class="toolbar clearfix">
<div class="right">
<div class="btn-group">
<button type="submit" class="btn btn-small btn-warning tip" data-original-title="Submit Aritcle">
<span class="icon-ok icon-white"></span>
</button>
<button type="button" class="btn btn-small btn-danger tip" data-original-title="Delete Article">
<span class="icon-remove icon-white"></span>
</button>
</div>
</div>
<div class="left">
<div style="font-size: 11pt; font-family: -webkit-body; font-weight: bolder;">Article Exchange Request</div>
</div>
</div>
<div class="row-form clearfix">
<div class="span3">Title</div>
<div class="span9"><input type="text" name="title" value="" placeholder="main subject title"></div>
</div>
<div class="row-form clearfix">
<div class="span3">First Sentence</div>
<div class="span9"><input type="text" name="s1" value="" placeholder="Sentence with max of 200 characters"></div>
</div>
<div class="row-form clearfix">
<div class="span3">Second Sentence</div>
<div class="span9"><input type="text" name="s2" value="" placeholder="Optional sentence with max of 200 characters"></div>
</div>
<div class="row-form clearfix">
<div class="span3">Third Sentence</div>
<div class="span9"><input type="text" name="s3" value="" placeholder="Optional Sentence with max of 200 characters"></div>
</div>
<div class="row-form clearfix">
<div class="span3">Website Url</div>
<div class="span9"><input type="text" name="link" value="" placeholder="Url to be included in articles"></div>
</div>
<div class="row-form clearfix">
<div class="span3">Website Url Title</div>
<div class="span9"><input type="text" name="link_text" value="" placeholder="Url text to be included in articles limit 150 characters"></div>
</div>
<div class="row-form clearfix">
<div class="span3">Credit Offer</div>
<div class="span9"><input type="text" name="cost_value" value="" placeholder="Example: 100"></div>
</div>
</div>
</form>
</div>
The problem is someone can just come along and submit the form without entering any data at all, and it still submits, and adds empty results to every column in the database. Meaning no NULL values are being set in the database.
My form processing
<?
if (isset($_POST[cost_value])) {
$stmt = $db->prepare("SELECT credits from member_credits WHERE username = :username");
$stmt->bindParam(':username', $username);
$stmt->execute();
$row = $stmt->fetch();
$count = $row[credits];
if ($count > $_POST[cost_value]) {
$stmt = $db->prepare('INSERT INTO article_requests (title, s1, s2, s3, link, link_text, offer, username) VALUES (:title, :s1, :s2, :s3, :link, :link_text, :offer, :username)');
$stmt->bindValue(':title', $_POST[title]);
$stmt->bindValue(':s1', $_POST[s1]);
$stmt->bindValue(':s2', $_POST[s2]);
$stmt->bindValue(':s3', $_POST[s3]);
$stmt->bindValue(':link', $_POST[link]);
$stmt->bindValue(':link_text', $_POST[link_text]);
$stmt->bindValue(':offer', $_POST[cost_value]);
$stmt->bindValue(':username', $username);
$stmt->execute();
}
}
?>
What I expect to happen on submission is if no value is entered in the form it should be set to NULL... that would mean if no $_POST[cost_value] was set, the form wouldn't submit. As it stands now, it is ALWAYS seeing $_POST[cost_value] as being set - sometimes to a value such as $_POST[cost_value] = '100'; and other times $_POST[cost_value] = ''; but in both cases, the form will submit. Equally I have the database set so that only the values of s2 and s3 should be able to be null, however when everything is submitting empty instead of null, thats mostly useless.
What exactly am I missing? Why will it not say $_POST[cost_value] = NULL; if the user has not input a value? Why is it submitting ''?? I have a feeling its going to be something basic I am overlooking, I just don't see it....
UPDATE FOR CLARITY
I know there are a number of ways to fix this. I can use if then tests, I can use temporary variables, I can check for the value itself is something other than ''. I know WHAT the work arounds are. I don't understand WHY they are needed. Why is it I make forms every day of the week using if isset, and everyday that works. If a form has no values input from the user, the form won't submit because an if isset returns false. So why is it on this particular form it is submitting '' instead of saying !isset don't process.

Answer 1

// Assign a var to the post.
$DIRTY_Var = $_POST['varinputname'];
// ensure the var is cleaned (no funny business when submitting)
$CLEAN_Var = filter_var($DIRTY_Var,` FILTER_SANITIZE_STRING);
// check if the var is empty, or has value. If empty it will set the var to null;
if (!$CLEAN_Var) { return $CLEAN_Var = null; }
// validate input
$valid = true;
// insert data
if ($valid) {
$pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$sql = "INSERT INTO Table ("
."ColumnNames"
.")"
."VALUES("
."?"
.")";
$q = $pdo->prepare($sql);
$q->execute(array($CLEAN_Var));
// Once INSERT is completed, we will redirect.
header("Location:" 'link.php'); exit;
}

Answer 2

Try
If(isset($_POST['cost_value']) && !empty($_POST['cost_value']){
// your code
}