PHP: Insert Data into MySQL

I had this question after viewing PHP: concatenate query. I have implemented this code in my project and it works beautifully. I do have one more follow up question.
The code to add the data to the database is:

//set up db connection
//http://php.net/manual/en/function.require-once.php
require_once('connection.php');
$connection = connect_db("localhost", "root", "xxxx", "tt284");


// COLUMNS IN TABLE
$elements = array
( 'RunnerID'
 , 'EventID'
 , 'Date'
 , 'FinishTime'
 , 'Position'
 , 'CategoryID'
 , 'AgeGrade'
 , 'PB'
)
;


// SANITIZE THE POST ARRAY
// my_espace_function is used to protect the site from SQL Injection 
// https://en.wikipedia.org/wiki/SQL_injection
foreach ($elements as $key)
{
    // KEEP ONLY THE ELEMENTS WE WANT
    $safe[$key] = !empty($_POST[$key]) ? mysql_real_escape_string($_POST[$key]) : "";
}

// CREATE THE COLUMN NAMES FROM THE LIST OF ELEMENTS
$cols = implode(', ', $elements);

// CREATE THE VALUES FROM THE SAFE COPY OF $_POST AND THE LIST OF ELEMENTS
$vals
= "'"
. implode("', '", $safe)
. "'"
;

// CREATE THE QUERY
$sql
= 'INSERT INTO RESULTS ('
. $cols
. ') VALUES ('
. $vals
. ')'
;


// SHOW THE REQUEST AND THE WORK PRODUCT
echo '<pre>';
print_r($_POST);
var_dump($sql);

	//mysql_query($sql, $connection, 'tt284');

		// RUN THE QUERY
		$res = mysql_query($sql, $connection);
		if (!$res) trigger_error('Query Failure:' . mysql_error(), E_USER_ERROR);

		echo "Success!";
			

	?>

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How do I change this code to catch PHP_Posts that are empty? For example:

If the value for 'Position', 'Category ID' or 'AgeGrade' fields is empty when the form is submitted then the value should be submitted as '-1'. Similarly, if no value is given for 'Personal best' then this should be submitted as '0'.

Thanks

mscola