[VarDumper] Backport handler lock when using VAR_DUMPER_FORMAT

Author: ogizanagi

VarDumper.php

@@ -53,6 +53,12 @@ public static function dump($var)
     public static function setHandler(callable $callable = null)
     {
         $prevHandler = self::$handler;
+
+        // Prevent replacing the handler with expected format as soon as the env var was set:
+        if (isset($_SERVER['VAR_DUMPER_FORMAT'])) {
+            return $prevHandler;
+        }
+
         self::$handler = $callable;
 
         return $prevHandler;