clients.php

<?php 

return array(
	'123456',
	'1234567',
	'12345678'
);

Permission.php

<?php

class Permission
{
	
	static $dictionaryPath = 'clients.php';

	/**
	 * Check if the clientId exists in the dictionary
	 * @param $clientId
	 * return (boolean) true if exists, false otherwise 
	 */
	public static function check($clientId) {
		return in_array($clientId, self::_getAvailableClients());
	}

	private static function _getAvailableClients(){
		return require __DIR__ . DIRECTORY_SEPERATOR . static::$dictionaryPath;
	}
}