Copying Row, Losing NULL value - php

Question

I have this working where it will copy the row, and then link the new row to the previous row.
Where my issue is, is in copying over the NULL values. When I run this all null values go into the new row as blank.
How would I get it to change the value to NULL if it was originally NULL?
$result = $mysqli->query("SELECT * FROM rdpricing WHERE psid = '$dupsid';");
if($result->num_rows >= "1"){
$count = $result->num_rows;
$cols = array();
$result = $mysqli->query("SHOW COLUMNS FROM rdpricing");
while ($r = $result->fetch_array(MYSQLI_ASSOC)) {
if (!in_array($r["Field"], array("rdpid", "psid", "rdold"))) { //Excluding these columns
$cols[] = $r["Field"];
}
}
// Build and do the insert
$result = $mysqli->query("SELECT * FROM rdpricing WHERE psid = '$dupsid';");
while ($r = $result->fetch_array(MYSQLI_ASSOC)) {
$insertSQL = "INSERT INTO rdpricing (" . implode(", ",$cols) . ", rdold) VALUES (";
$count = count($cols);
foreach($cols as $counter=>$col) {
**// This is where I Believe it needs to happen, and what I have attempted, and it is NOT working**
if(empty($r[$col]) || is_null($r[$col]) || $r[$col] == ""){
$r[$col] = NULL;
}
$insertSQL .= "'" . $mysqli->real_escape_string($r[$col]) . "'";
if ($counter < ($count - 1)) {
$insertSQL .= ", ";
}
} // END foreach
$insertSQL .= ", '".$r["rdpid"]."');";
$mysqli->query($insertSQL);
if ($mysqli->affected_rows < 1) {
printf("%s\n", $mysqli->error);
} else {
}
$new_id = $mysqli->insert_id;
$statement = $mysqli->prepare("UPDATE rdpricing SET `psid`=? WHERE `rdpid`=?");
$statement->bind_param('ss', $new_psid, $new_id);
// Execute the prepared query.
$statement->execute();
$statement->close();
}
}

Answer 1

Generated from info in the comments:
#reset/create before the foreach, create an empty array
$insertSQLValues=array();
#in the foreach do some on given type
if(is_null($r[$col])){#real null
$r[$col] = "null";
} else if (empty($r[$col]) || $r[$col] == ""){#empty values
$r[$col] = "''";
} else {#standart data
$r[$col] = "'".$mysqli->real_escape_string($r[$col])."'";
}
$insertSQLValues[]=$r[$col];
#later
$insertSQL .= implode(', ',$insertSQLValues).", '".$r["rdpid"]."');";
Hopefully you can merge that into your code.