ThinkPHP POST is invalid. how can this problem be solved?

ThinkPHP POST does not display the page according to The POST value, but it does not seem to get the POST value: $ username & nbsp ;=& nbsp; $ this-& gt; $ _ POST ['username']; // $ name & nbsp; = & nbsp; $ this-& gt; _ post (ThinkPHP POST is invalid
The page is displayed based on the value of POST, but the value of POST cannot be obtained:

$ Username = $ this-> $ _ POST ['username'];
// $ Name = $ this-> _ post ('name'); this industry is invalid.
$ User = M ('user ');
$ Data = $ user-> where ('username = "'. $ username.'" ')-> field ('Id, username')-> find ();
If ($ data ){
$ This-> success ('success ');
} Else {
$ This-> error ('failed ');
}

But the _ param method is successful. Please explain it.
------ Solution --------------------
Isn't your post parameter username?

$ This-> _ post ('name'); of course it is invalid, $ this-> _ post ('Username'); try again
------ Solution --------------------
Dump ($ _ POST); give it a try