Php - Validations and errors - unique name - php

Question

I don't want to save duplicate name record. I want to display errors back to the user.
But it doesn't work. I don't know what I'm doing wrong.
protected function has_unique_name($value, $current_id="0") {
$sql = "SELECT * FROM photographs ";
$sql .= "WHERE caption='" . self::$database->escape_string($this->caption) . "' ";
$sql .= "AND id != '" . self::$database->escape_string($current_id) . "'";
echo $sql;
$result = self::$database->query($sql);
$products_count = $result->num_rows;
echo $products_count . "<br />" ;
$result->free();
return $products_count === 0;
}
protected function validate() {
$this->errors = [];
$value = $this->caption;
$current_id = isset($this->id) ? $this->id : '0';
if(!$this->has_unique_name($this->caption, $current_id)) {
$errors[] = "The name must be unique.";
}
return $this->errors;
}
public function create() {
$this->validate();
if(!empty($this->errors)) { return false; }
...