title: Need help regarding custom authentication firebase + php
body: <p>I am using firebase SDK 3.0. I need help regarding custom authentication.
  I am doing custom authentication with firebase. For generating token I am using php server I am able to generate token successfully with HS256 algorithm
  but when i am using that token to authenticate with firebase I am getting like this</p>
  
  <pre><code>{"code":"auth/invalid-custom-token","message":"The custom token format is incorrect. Please check the documentation."}
  </code></pre>
  
  <p>response at network</p>
  
  <pre><code>{
   "error": {
    "errors": [
     {
      "domain": "global",
      "reason": "invalid",
      "message": "INVALID_CUSTOM_TOKEN"
     }
    ],
    "code": 400,
    "message": "INVALID_CUSTOM_TOKEN"
   }
  }
  </code></pre>
  
  <p>can anyone help me to out from this</p>
  
  <p>Thanks</p>