juju-php/PDO/filter.class.php

<?php
	namespace Juju\PDO;

	class Filter {
		private static $filters = [];
		private $name;
		private $filter;
		private function __construct(string $name, string $filter){
			$this->name = $name;
			self::$filters[$name] = $this;
		}
		public function name(){
			return $this->name;
		}
		public function __toString(){
			return $this->filter;
		}
		public static function named(string $name){
			if(!isset(self::$filters[$name])){
				throw new \Exception("Filter {$name} is not defined");
			}
			return self::$filters[$name];
		}
		public static function add(string $name, string $filter){
			if(isset(self::$filters[$name])){
				throw new \Exception("Filter {$name} is already defined");
			}
			return new  Filter($name, $filter);
		}
	}
	Filter::add('not null', 'is not null');
	Filter::add('null', 'is null');
?>