example.php

<?php

class Example
{
  /** @var int */
  private $amount;
  
  public function __construct(int $amount)
  {
    $this->amount = $amount;
  }
  
  public function execute(): string
  {
    if ($this->amount === 0) {
      throw new \InvalidArgumentException('O valor passado precisa ser maior que zero!');
    } else if ($this->amount > 1000) {
      throw new \InvalidArgumentException('O valor passado precisa ser menor que mill!');
    } else if ($this->amount < 456) {
      return "Amount is less than 456";
    } else {
      $db = Database::prepare('SELECT amount FROM item_values;');
      $stmt = $db->execute();
      $values = $stmt->fetchAll();
      
      $oldValues = array_reduce($values, function ($acc, $value) {
        return $acc + $vale->amount;
      }, 0);
      
      if ($oldValues < $this->amount) {
        throw new \InvalidArgumentException(sprintf(
          'O valor passado deve ser menor que %s!',
          $oldValues
        ));
      }

      return "Amount is equal or greather than 456";
    }
  }
}