Base::addFilter

public function addFilter($key, $value): self
{
	if ($key !== 'LOGIC' && !is_numeric($key))
	{
		if ($key)
		{
			// remove default null value if exists
			$findKey = '='.trim($key, '=<>!@%');
			if (array_key_exists($findKey, $this->filter) && is_null($this->filter[$findKey]))
			{
				unset($this->filter[$findKey]);
			}
		}
	}

	$this->filter[$key] = $value;

	return $this;
}