This time I will tell you about the range of HTTP errors that are starting with 4. HTTP errors that start with 4 indicate there is a problem or error at the client or user agent end. Most common are:

400 Bad Request:
The request could not be understood by the server due to malformed syntax. Pay attenting to request string (usually GET)

401 Unauthorized:
The request requires user authorization (such as through htaccess) but the authorization codes sent were invalid or the user was not recognized in the system. This error is sent when the username sent is not recognized and when the username and password combination are incorrect.

403 Forbidden:
The server understood the request, but refuses to fullfill it. Often may appear when directory listing is denied.

404 Not Found:
This is the most easily recognized error message. It states that the URI requested does not exist on the server.

405 Method Not Allowed:
The method specified is not allowed for the resource requested. Check your GET and POST variables.

406 Not Acceptable:
The resource identified by the request is only capable of generating response entities which have content characteristics not acceptable according to the accept headers sent in the request.

407 Proxy Authentication Required:

This is similar to 401, but the client must first authenticate with a proxy.

408 Request Timeout:
The client did not produce a request within the time that the server was prepared to wait.

410 Gone:
The resource requested was once on this server but is no longer here and there is no redirect in place for it.

411 Length Required:
The server requires a content-length sent with the request.

414 Request-URI Too Long:
The server has a limit as to the size of a URI.

415 Unsupported Media Type:
The entity of the request is a format not supported by the requested resource.

These are most popular status codes, that I’ve seen while testing my scripts and looking at my servers. Check W3C for the complete list if you need. Wish you to see these errors more rarely!

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