PHP OOP binding problem inside the method - php

Question

I try so hard to create this method with prepare, bind, and execute inside the class. But I guess I don't have enough knowledge about it whatever I do I couldn't make it work. I'm looking google for hours. I know that below code is wrong about binding but Can someone show me the correct way to do binding inside this method?
class User {
protected static $db_table = "users";
protected static $db_table_fields = array('username', 'password', 'first_name', 'last_name');
public $id;
public $username;
public $password;
public $first_name;
public $last_name;
protected function properties() {
$properties = array();
foreach (static::$db_table_fields as $db_field) {
if (property_exists($this, $db_field)) {
$properties[$db_field] = $this->$db_field;
}
}
return $properties;
}
protected function clean_properties() {
global $database;
$clean_properties = array();
foreach ($this->properties() as $key => $value) {
$clean_properties[$key] = $value;
}
return $clean_properties;
}
public function create($params= []){
global $database;
$properties = $this->clean_properties();
$fields = ":" . implode("',:'", static::$db_table_fields);
$sql= "INSERT INTO " .static::$db_table . "(" . implode(",", array_keys($properties)) . ")
VALUES('". $fields ."')";
$stmt = $database->prepare($sql);
foreach ($fields as $field => &$params) {
$stmt->bindValue($field, $params);
}
if ($stmt->execute()) {
$this->id = $database->InsertId();
return true;
} else {
return false;
}
}

Answer 1

So I made my code like this for binding. I don't know if it is the correct way. I know I keep changing stuff. I guess it is the best way to learn to try different stuff.
//$param = array('x'=>$x, 'y'=>$y);
public function bind($param = [], $type=NULL){
foreach($param as $key=>&$value){
if (is_null($type)) {
switch (true) {
case is_int($value):
$type = PDO::PARAM_INT;
break;
case is_bool($value):
$type = PDO::PARAM_BOOL;
break;
case is_null($value):
$type = PDO::PARAM_NULL;
break;
default:
$type = PDO::PARAM_STR;
}
}
$this->stmt->bindValue(':'.$key,$value);
}
return $this->stmt;
}