Throws an ConversionException if the json encoding fails

lib/Doctrine/DBAL/Types/ConversionException.php

@@ -99,4 +99,16 @@ class ConversionException extends \Doctrine\DBAL\DBALException
             implode(', ', $possibleTypes)
         ));
     }
+
+    static public function conversionFailedSerialization($value, $format, $error, \Exception $previous = null)
+    {
+        $actualType = is_object($value) ? get_class($value) : gettype($value);
+
+        return new self(sprintf(
+            "Could not convert PHP type '%s' to '%s', as an '%s' error was triggered by the serialization",
+            $actualType,
+            $format,
+            $error
+        ));
+    }
 }

lib/Doctrine/DBAL/Types/JsonType.php

@@ -46,7 +46,13 @@ class JsonType extends Type
             return null;
         }
 
-        return json_encode($value);
+        $encoded = json_encode($value);
+
+        if (JSON_ERROR_NONE !== json_last_error()) {
+            throw ConversionException::conversionFailedSerialization($value, 'json', $this->getLastErrorMessage());
+        }
+
+        return $encoded;
     }
 
     /**
@@ -91,4 +97,44 @@ class JsonType extends Type
         //return ! $platform->hasNativeJsonType();
         return true;
     }
+
+    /**
+     * Get the latest json error message
+     *
+     * This method declaration has been extracted from symfony's php 5.5 polyfill
+     *
+     * @link https://github.com/symfony/polyfill-php55/blob/master/Php55.php
+     * @link http://nl1.php.net/manual/en/function.json-last-error-msg.php
+     *
+     * @return string
+     */
+    private function getLastErrorMessage()
+    {
+        if (function_exists('json_last_error_msg')) {
+            return json_last_error_msg();
+        }
+
+        switch (json_last_error()) {
+            case JSON_ERROR_NONE:
+                return 'No error';
+
+            case JSON_ERROR_DEPTH:
+                return 'Maximum stack depth exceeded';
+
+            case JSON_ERROR_STATE_MISMATCH:
+                return 'State mismatch (invalid or malformed JSON)';
+
+            case JSON_ERROR_CTRL_CHAR:
+                return 'Control character error, possibly incorrectly encoded';
+
+            case JSON_ERROR_SYNTAX:
+                return 'Syntax error';
+
+            case JSON_ERROR_UTF8:
+                return 'Malformed UTF-8 characters, possibly incorrectly encoded';
+
+            default:
+                return 'Unknown error';
+        }
+    }
 }