登录功能似乎无法正常运行

I've been sitting here pondering this for a good 30 minutes now and I just can't see what's wrong.

I've added prints but it just doesn't seem to be contacting the database correctly or not pulling the data from it correctly.

$conf['sql_host'] = 'localhost';
$conf['sql_user'] = 'root';
$conf['sql_pass'] = '';
$conf['sql_data'] = 'c_webauth';

    mysql_connect( $conf['sql_host'], $conf['sql_user'], $conf['sql_pass'] ) or die( 'Connection failed: '.mysql_error() );
    mysql_select_db( $conf['sql_data'] );



function login( $user, $pass ){

    session_regenerate_id();
        if ( isset( $_SESSION['user_id'] ) ) {
            unset( $_SESSION['user_id'] );
        }

        $qry = mysql_query( "SELECT user_id, user, user_group, user_name FROM c_users where user='$user' AND pass='".md5($pass)."'" );

        if ( mysql_num_rows( $qry ) > 0 ) {
            session_regenerate_id();

            while ( $data = mysql_fetch_array( $qry ) ) {
                $_SESSION['user_id'] = $data['user_id'];
                $_SESSION['user_group'] = $data['user_group'];
                $_SESSION['user'] = $data['user'];
                $_SESSION['user_name'] = $data['user_name'];

                // Debug
                echo $_SESSION['user_id'];
                echo $_SESSION['user_group'];
                echo $_SESSION['user'];
                echo $_SESSION['user_name'];

                session_start( ); 
            }
        }
}

login( "username", "password" );

Anychance you can see whats wrong?

Everyone has mentioned that I had my session_start(); in the wrong place, but the echo's print nothing, absolutely no errors anywhere.

You are calling session_start() at the end instead before doing session checks

session_start( ); it should used before session use

You have put session_start( ) at bottom , after you have assigned SESSION variables

Put this function on top of the page and remove from bottom.

You need to put session_start() at the top of your code, because its at the bottom all $_SESSION variables are undeclared so they wont return a value. One session_start() is at the top you'll get the global $_SESSION variables and you'll also be able to set them. I advise that you create a script called config.php that has session_start() and you can include it in all your scripts instead of add session_start() everytime.
Hope this helps you out :D

<?php
session_start();

$conf['sql_host'] = 'localhost';
$conf['sql_user'] = 'root';
$conf['sql_pass'] = '';
$conf['sql_data'] = 'c_webauth';

    mysql_connect( $conf['sql_host'], $conf['sql_user'], $conf['sql_pass'] ) or die( 'Connection failed: '.mysql_error() );
    mysql_select_db( $conf['sql_data'] );



function login( $user, $pass ){

    session_regenerate_id();
        if ( isset( $_SESSION['user_id'] ) ) {
            unset( $_SESSION['user_id'] );
        }

        $qry = mysql_query( "SELECT user_id, user, user_group, user_name FROM c_users where user='$user' AND pass='".md5($pass)."'" );

        if ( mysql_num_rows( $qry ) > 0 ) {
            session_regenerate_id();

            while ( $data = mysql_fetch_array( $qry ) ) {
                $_SESSION['user_id'] = $data['user_id'];
                $_SESSION['user_group'] = $data['user_group'];
                $_SESSION['user'] = $data['user'];
                $_SESSION['user_name'] = $data['user_name'];

                // Debug
                echo $_SESSION['user_id'];
                echo $_SESSION['user_group'];
                echo $_SESSION['user'];
                echo $_SESSION['user_name'];


            }
        }
}

login( "username", "password" );
?>

I've fixed your code for you.