php - mySQL database not updating from form -

not sure error is. in html have form, , when click submit gives me error. connection working, reason won't add database

 <?php      if( isset( $_post['submit'] ) ){         $username = $_post['username'];         $password = $_post['password'];         $connection = mysqli_connect( 'localhost', 'root', '', 'loginapp' );          if($connection){             echo "we connected";         }         else{             die( "connection failed" );         }          $query = "insert users(username,password) ";         $query.= "values('$username','$password')";         $result = mysqli_query( $connection, $query );          if( !$result ){ // if not true, put query failed             die('query failed' .mysqli_error($connection));         }     }      ?> 

i wondering if there wrong here code isnt working planned

  $query = "insert users(username,password) ";   $query.= "values('$username','$password')";   $result = mysqli_query( $connection, $query );    if( !$result ){ // if not true, put query failed       die( 'query failed' . mysqli_error( $connection ) );   }   } 

you trying insert duplicate entry table. username column. can around either providing value username not in table yet, or delete record table can re-insert it. can more specific if include definition users table in question, basic problem having.

if table defined numeric typed id column primary key, , not specify new value in insert, this:

insert users (id, username, password) values (1, 'joeuser', 'password'); 

instead, doing have in question:

insert users (username, password) values ('joeuser', 'password'); 

the record inserted either have null or 0 default insert value. each time insert performed same value inserted id column, cause duplicate value error getting.

to around that, defining id column auto-increment tells anytime perform insert, , not specify value id use next available integer default value.

the other approach specify new value id on each execute of insert statement, so:

insert users (id, username, password) values (1, 'joeuser', 'password');  insert users (id, username, password) values (2, 'daveuser', 'password');  insert users (id, username, password) values (3, 'janeuser', 'password'); 

for table this, using auto-increment best way go.