How do you pass something like "+ interval 4" to the bindParam - php

Question

I am trying to pass a variable that looks like this "+ interval 4" or "- interval 5" to this PDO statement.
$query = "INSERT INTO
" . $this->table_name . " (employee_id, work_date)
SELECT DISTINCT emp_num,
CURDATE() :test DAY FROM " . $this->table_name3 . "";
My bindParam
$stmt->bindParam(":test", $this->interval);
I not sure if I should be using bindValue having a hard time figuring this out.
This works fine but i know it is not right
$query = "INSERT INTO
" . $this->table_name . " (employee_id, work_date)
SELECT DISTINCT emp_num,
CURDATE() " . $this->interval . " DAY FROM " . $this->table_name3 . "";
and here is the whole function if that helps
function sqlbuild(){
$query = "INSERT INTO
" . $this->table_name . " (employee_id, work_date)
SELECT DISTINCT emp_num,
CURDATE() " . $this->interval . " DAY FROM " . $this->table_name3 . "";
$stmt = $this->conn->prepare( $query );
echo json_encode($stmt);
$stmt->bindParam(":test", $this->interval);
//$stmt->bindParam(1, $this->interval);
echo json_encode($this-> interval);
if($stmt->execute()){
return true;
}else{
return false;
}
}