--- /dev/null
+<?php
+
+/*
+ * This file is a part of dflydev/placeholder-resolver.
+ *
+ * (c) Dragonfly Development Inc.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Dflydev\PlaceholderResolver\DataSource;
+
+interface DataSourceInterface
+{
+ /**
+ * Does specified key exist?
+ *
+ * If $system is true, check system data. Otherwise,
+ * use application data. (think system == $_SERVER)
+ *
+ * @param string $key
+ * @param bool|null $system
+ *
+ * @return bool
+ */
+ public function exists($key, $system = false);
+
+ /**
+ * Get key's value
+ *
+ * If $system is true, check system data. Otherwise,
+ * use application data. (think system == $_SERVER)
+ *
+ * @param string $key
+ * @param bool|null $system
+ *
+ * @return string|null
+ */
+ public function get($key, $system = false);
+}