diff --git a/src/Symfony/Component/Cache/Adapter/ArrayAdapter.php b/src/Symfony/Component/Cache/Adapter/ArrayAdapter.php
index 07fc198115a5..1c272c76ab3c 100644
--- a/src/Symfony/Component/Cache/Adapter/ArrayAdapter.php
+++ b/src/Symfony/Component/Cache/Adapter/ArrayAdapter.php
@@ -56,22 +56,10 @@ function ($key, $value, $isHit) use ($defaultLifetime) {
      */
     public function getItem($key)
     {
-        $isHit = $this->hasItem($key);
-        try {
-            if (!$isHit) {
-                $this->values[$key] = $value = null;
-            } elseif (!$this->storeSerialized) {
-                $value = $this->values[$key];
-            } elseif ('b:0;' === $value = $this->values[$key]) {
-                $value = false;
-            } elseif (false === $value = unserialize($value)) {
-                $this->values[$key] = $value = null;
-                $isHit = false;
-            }
-        } catch (\Exception $e) {
-            CacheItem::log($this->logger, 'Failed to unserialize key "{key}"', array('key' => $key, 'exception' => $e));
+        if (!$isHit = $this->hasItem($key)) {
             $this->values[$key] = $value = null;
-            $isHit = false;
+        } else {
+            $value = $this->storeSerialized ? $this->unfreeze($key, $isHit) : $this->values[$key];
         }
         $f = $this->createCacheItem;
 
@@ -84,7 +72,9 @@ public function getItem($key)
     public function getItems(array $keys = array())
     {
         foreach ($keys as $key) {
-            CacheItem::validateKey($key);
+            if (!\is_string($key) || !isset($this->expiries[$key])) {
+                CacheItem::validateKey($key);
+            }
         }
 
         return $this->generateItems($keys, microtime(true), $this->createCacheItem);
@@ -120,15 +110,8 @@ public function save(CacheItemInterface $item)
 
             return true;
         }
-        if ($this->storeSerialized) {
-            try {
-                $value = serialize($value);
-            } catch (\Exception $e) {
-                $type = is_object($value) ? get_class($value) : gettype($value);
-                CacheItem::log($this->logger, 'Failed to save key "{key}" ({type})', array('key' => $key, 'type' => $type, 'exception' => $e));
-
-                return false;
-            }
+        if ($this->storeSerialized && null === $value = $this->freeze($value)) {
+            return false;
         }
         if (null === $expiry && 0 < $item["\0*\0defaultLifetime"]) {
             $expiry = microtime(true) + $item["\0*\0defaultLifetime"];
diff --git a/src/Symfony/Component/Cache/Simple/ArrayCache.php b/src/Symfony/Component/Cache/Simple/ArrayCache.php
index 6cef8b6e745c..14f35bd6a901 100644
--- a/src/Symfony/Component/Cache/Simple/ArrayCache.php
+++ b/src/Symfony/Component/Cache/Simple/ArrayCache.php
@@ -45,9 +45,20 @@ public function __construct(int $defaultLifetime = 0, bool $storeSerialized = tr
      */
     public function get($key, $default = null)
     {
-        foreach ($this->getMultiple(array($key), $default) as $v) {
-            return $v;
+        if (!\is_string($key) || !isset($this->expiries[$key])) {
+            CacheItem::validateKey($key);
+        }
+        if (!$isHit = isset($this->expiries[$key]) && ($this->expiries[$key] > microtime(true) || !$this->delete($key))) {
+            $this->values[$key] = null;
+
+            return $default;
+        }
+        if (!$this->storeSerialized) {
+            return $this->values[$key];
         }
+        $value = $this->unfreeze($key, $isHit);
+
+        return $isHit ? $value : $default;
     }
 
     /**
@@ -61,7 +72,9 @@ public function getMultiple($keys, $default = null)
             throw new InvalidArgumentException(sprintf('Cache keys must be array or Traversable, "%s" given', is_object($keys) ? get_class($keys) : gettype($keys)));
         }
         foreach ($keys as $key) {
-            CacheItem::validateKey($key);
+            if (!\is_string($key) || !isset($this->expiries[$key])) {
+                CacheItem::validateKey($key);
+            }
         }
 
         return $this->generateItems($keys, microtime(true), function ($k, $v, $hit) use ($default) { return $hit ? $v : $default; });
@@ -87,7 +100,9 @@ public function deleteMultiple($keys)
      */
     public function set($key, $value, $ttl = null)
     {
-        CacheItem::validateKey($key);
+        if (!\is_string($key)) {
+            CacheItem::validateKey($key);
+        }
 
         return $this->setMultiple(array($key => $value), $ttl);
     }
@@ -103,27 +118,20 @@ public function setMultiple($values, $ttl = null)
         $valuesArray = array();
 
         foreach ($values as $key => $value) {
-            \is_int($key) || CacheItem::validateKey($key);
+            if (!\is_int($key) && !(\is_string($key) && isset($this->expiries[$key]))) {
+                CacheItem::validateKey($key);
+            }
             $valuesArray[$key] = $value;
         }
         if (false === $ttl = $this->normalizeTtl($ttl)) {
             return $this->deleteMultiple(array_keys($valuesArray));
         }
-        if ($this->storeSerialized) {
-            foreach ($valuesArray as $key => $value) {
-                try {
-                    $valuesArray[$key] = serialize($value);
-                } catch (\Exception $e) {
-                    $type = is_object($value) ? get_class($value) : gettype($value);
-                    CacheItem::log($this->logger, 'Failed to save key "{key}" ({type})', array('key' => $key, 'type' => $type, 'exception' => $e));
-
-                    return false;
-                }
-            }
-        }
         $expiry = 0 < $ttl ? microtime(true) + $ttl : PHP_INT_MAX;
 
         foreach ($valuesArray as $key => $value) {
+            if ($this->storeSerialized && null === $value = $this->freeze($value)) {
+                return false;
+            }
             $this->values[$key] = $value;
             $this->expiries[$key] = $expiry;
         }
diff --git a/src/Symfony/Component/Cache/Tests/Adapter/ArrayAdapterTest.php b/src/Symfony/Component/Cache/Tests/Adapter/ArrayAdapterTest.php
index 9503501899b3..7b65061cd12b 100644
--- a/src/Symfony/Component/Cache/Tests/Adapter/ArrayAdapterTest.php
+++ b/src/Symfony/Component/Cache/Tests/Adapter/ArrayAdapterTest.php
@@ -36,12 +36,12 @@ public function testGetValuesHitAndMiss()
 
         // Hit
         $item = $cache->getItem('foo');
-        $item->set('4711');
+        $item->set('::4711');
         $cache->save($item);
 
         $fooItem = $cache->getItem('foo');
         $this->assertTrue($fooItem->isHit());
-        $this->assertEquals('4711', $fooItem->get());
+        $this->assertEquals('::4711', $fooItem->get());
 
         // Miss (should be present as NULL in $values)
         $cache->getItem('bar');
@@ -50,7 +50,7 @@ public function testGetValuesHitAndMiss()
 
         $this->assertCount(2, $values);
         $this->assertArrayHasKey('foo', $values);
-        $this->assertSame(serialize('4711'), $values['foo']);
+        $this->assertSame(serialize('::4711'), $values['foo']);
         $this->assertArrayHasKey('bar', $values);
         $this->assertNull($values['bar']);
     }
diff --git a/src/Symfony/Component/Cache/Traits/ArrayTrait.php b/src/Symfony/Component/Cache/Traits/ArrayTrait.php
index 86ad17e9c52e..9cedb8d2cc50 100644
--- a/src/Symfony/Component/Cache/Traits/ArrayTrait.php
+++ b/src/Symfony/Component/Cache/Traits/ArrayTrait.php
@@ -34,7 +34,21 @@ trait ArrayTrait
      */
     public function getValues()
     {
-        return $this->values;
+        if (!$this->storeSerialized) {
+            return $this->values;
+        }
+
+        $values = $this->values;
+        foreach ($values as $k => $v) {
+            if (null === $v || 'N;' === $v) {
+                continue;
+            }
+            if (!\is_string($v) || !isset($v[2]) || ':' !== $v[1]) {
+                $values[$k] = serialize($v);
+            }
+        }
+
+        return $values;
     }
 
     /**
@@ -42,9 +56,12 @@ public function getValues()
      */
     public function hasItem($key)
     {
+        if (\is_string($key) && isset($this->expiries[$key]) && $this->expiries[$key] > microtime(true)) {
+            return true;
+        }
         CacheItem::validateKey($key);
 
-        return isset($this->expiries[$key]) && ($this->expiries[$key] > microtime(true) || !$this->deleteItem($key));
+        return isset($this->expiries[$key]) && !$this->deleteItem($key);
     }
 
     /**
@@ -62,8 +79,9 @@ public function clear()
      */
     public function deleteItem($key)
     {
-        CacheItem::validateKey($key);
-
+        if (!\is_string($key) || !isset($this->expiries[$key])) {
+            CacheItem::validateKey($key);
+        }
         unset($this->values[$key], $this->expiries[$key]);
 
         return true;
@@ -80,21 +98,10 @@ public function reset()
     private function generateItems(array $keys, $now, $f)
     {
         foreach ($keys as $i => $key) {
-            try {
-                if (!$isHit = isset($this->expiries[$key]) && ($this->expiries[$key] > $now || !$this->deleteItem($key))) {
-                    $this->values[$key] = $value = null;
-                } elseif (!$this->storeSerialized) {
-                    $value = $this->values[$key];
-                } elseif ('b:0;' === $value = $this->values[$key]) {
-                    $value = false;
-                } elseif (false === $value = unserialize($value)) {
-                    $this->values[$key] = $value = null;
-                    $isHit = false;
-                }
-            } catch (\Exception $e) {
-                CacheItem::log($this->logger, 'Failed to unserialize key "{key}"', array('key' => $key, 'exception' => $e));
+            if (!$isHit = isset($this->expiries[$key]) && ($this->expiries[$key] > $now || !$this->deleteItem($key))) {
                 $this->values[$key] = $value = null;
-                $isHit = false;
+            } else {
+                $value = $this->storeSerialized ? $this->unfreeze($key, $isHit) : $this->values[$key];
             }
             unset($keys[$i]);
 
@@ -105,4 +112,53 @@ private function generateItems(array $keys, $now, $f)
             yield $key => $f($key, null, false);
         }
     }
+
+    private function freeze($value)
+    {
+        if (null === $value) {
+            return 'N;';
+        }
+        if (\is_string($value)) {
+            // Serialize strings if they could be confused with serialized objects or arrays
+            if ('N;' === $value || (isset($value[2]) && ':' === $value[1])) {
+                return serialize($value);
+            }
+        } elseif (!\is_scalar($value)) {
+            try {
+                $serialized = serialize($value);
+            } catch (\Exception $e) {
+                $type = is_object($value) ? get_class($value) : gettype($value);
+                CacheItem::log($this->logger, 'Failed to save key "{key}" ({type})', array('key' => $key, 'type' => $type, 'exception' => $e));
+
+                return;
+            }
+            // Keep value serialized if it contains any objects or any internal references
+            if ('C' === $serialized[0] || 'O' === $serialized[0] || preg_match('/;[OCRr]:[1-9]/', $serialized)) {
+                return $serialized;
+            }
+        }
+
+        return $value;
+    }
+
+    private function unfreeze(string $key, bool &$isHit)
+    {
+        if ('N;' === $value = $this->values[$key]) {
+            return null;
+        }
+        if (\is_string($value) && isset($value[2]) && ':' === $value[1]) {
+            try {
+                $value = unserialize($value);
+            } catch (\Exception $e) {
+                CacheItem::log($this->logger, 'Failed to unserialize key "{key}"', array('key' => $key, 'exception' => $e));
+                $value = false;
+            }
+            if (false === $value) {
+                $this->values[$key] = $value = null;
+                $isHit = false;
+            }
+        }
+
+        return $value;
+    }
 }