Add return types

This is a breaking change since implementations without return types are considered to be "widening" them, which is not allowed in PHP.

Implementations that implement v3, however, are also implementing v1 and v2 as of PHP 7.4, so such implementations can use a require statement of `^1|^2|^3`.  Usage libraries also can use a require statement of `^1|^2|^3`

composer.json

@@ -19,7 +19,7 @@
     },
     "extra": {
         "branch-alias": {
-            "dev-master": "2.0.x-dev"
+            "dev-master": "3.0.x-dev"
         }
     }
 }

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(string $key, mixed $default = null);
+    public function get(string $key, mixed $default = null): mixed;
 
     /**
      * Persists data in the cache, uniquely referenced by a key with an optional expiration TTL time.
@@ -31,7 +31,7 @@ public function get(string $key, mixed $default = null);
      * @throws \Psr\SimpleCache\InvalidArgumentException
      *   MUST be thrown if the $key string is not a legal value.
      */
-    public function set(string $key, mixed $value, null|int|\DateInterval $ttl = null);
+    public function set(string $key, mixed $value, null|int|\DateInterval $ttl = null): bool;
 
     /**
      * Delete an item from the cache by its unique key.
@@ -43,14 +43,14 @@ public function set(string $key, mixed $value, null|int|\DateInterval $ttl = nul
      * @throws \Psr\SimpleCache\InvalidArgumentException
      *   MUST be thrown if the $key string is not a legal value.
      */
-    public function delete(string $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.
@@ -64,7 +64,7 @@ public function clear();
      *   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 getMultiple(iterable $keys, mixed $default = null);
+    public function getMultiple(iterable $keys, mixed $default = null): iterable;
 
     /**
      * Persists a set of key => value pairs in the cache, with an optional TTL.
@@ -80,7 +80,7 @@ public function getMultiple(iterable $keys, mixed $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(iterable $values, null|int|\DateInterval $ttl = null);
+    public function setMultiple(iterable $values, null|int|\DateInterval $ttl = null): bool;
 
     /**
      * Deletes multiple cache items in a single operation.
@@ -93,7 +93,7 @@ public function setMultiple(iterable $values, null|int|\DateInterval $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(iterable $keys);
+    public function deleteMultiple(iterable $keys): bool;
 
     /**
      * Determines whether an item is present in the cache.
@@ -110,5 +110,5 @@ public function deleteMultiple(iterable $keys);
      * @throws \Psr\SimpleCache\InvalidArgumentException
      *   MUST be thrown if the $key string is not a legal value.
      */
-    public function has(string $key);
+    public function has(string $key): bool;
 }