diff --git a/composer.json b/composer.json
index 2978fa5..da96ca8 100644
--- a/composer.json
+++ b/composer.json
@@ -10,7 +10,7 @@
         }
     ],
     "require": {
-        "php": ">=5.3.0"
+        "php": ">=7.1.0"
     },
     "autoload": {
         "psr-4": {
diff --git a/src/CacheInterface.php b/src/CacheInterface.php
index 99e8d95..39a079f 100644
--- a/src/CacheInterface.php
+++ b/src/CacheInterface.php
@@ -15,7 +15,7 @@ interface CacheInterface
      * @throws \Psr\SimpleCache\InvalidArgumentException
      *   MUST be thrown if the $key string is not a legal value.
      */
-    public function get($key, $default = null);
+    public function get(string $key, $default = null);
 
     /**
      * Persists data in the cache, uniquely referenced by a key with an optional expiration TTL time.
@@ -31,7 +31,7 @@ public function get($key, $default = null);
      * @throws \Psr\SimpleCache\InvalidArgumentException
      *   MUST be thrown if the $key string is not a legal value.
      */
-    public function set($key, $value, $ttl = null);
+    public function set(string $key, $value, $ttl = null): bool;
 
     /**
      * Delete an item from the cache by its unique key.
@@ -43,14 +43,14 @@ public function set($key, $value, $ttl = null);
      * @throws \Psr\SimpleCache\InvalidArgumentException
      *   MUST be thrown if the $key string is not a legal value.
      */
-    public function delete($key);
+    public function delete(string $key): bool;
 
     /**
      * Wipes clean the entire cache's keys.
      *
      * @return bool True on success and false on failure.
      */
-    public function clear();
+    public function clear(): bool;
 
     /**
      * Obtains multiple cache items by their unique keys.
@@ -80,7 +80,7 @@ public function getMultiple($keys, $default = null);
      *   MUST be thrown if $values is neither an array nor a Traversable,
      *   or if any of the $values are not a legal value.
      */
-    public function setMultiple($values, $ttl = null);
+    public function setMultiple($values, $ttl = null): bool;
 
     /**
      * Deletes multiple cache items in a single operation.
@@ -93,7 +93,7 @@ public function setMultiple($values, $ttl = null);
      *   MUST be thrown if $keys is neither an array nor a Traversable,
      *   or if any of the $keys are not a legal value.
      */
-    public function deleteMultiple($keys);
+    public function deleteMultiple($keys): bool;
 
     /**
      * Determines whether an item is present in the cache.
@@ -110,5 +110,5 @@ public function deleteMultiple($keys);
      * @throws \Psr\SimpleCache\InvalidArgumentException
      *   MUST be thrown if the $key string is not a legal value.
      */
-    public function has($key);
+    public function has(string $key): bool;
 }