PHP mysqli_select_db参数错误求助及代码问题排查

Hey there, let's tackle your problems step by step—since you're short on time and not a PHP pro, I'll keep this clear and actionable.

First, let's address the warning you're seeing:

Warning: mysqli_select_db() expects parameter 1 to be mysqli, string given in C:\wamp\www\trynew\down_line.php on line 50

This is a simple parameter order mix-up. The mysqli_select_db() function expects the database connection object first, then the database name—you had them reversed.

Beyond that, there are a few other critical issues in your code that are causing functionality problems and security risks. Let's fix them one by one:

Key Fixes & Explanations #

• Fix mysqli_select_db() order: Swap the arguments to mysqli_select_db($db, "testing");
• Add connection to mysqli_query: mysqli_query() requires the database connection as the first parameter—use mysqli_query($db, $query);
• Fix Session assignment syntax: Your current $row.$_SESSION['fullname'] = $Fname; is backwards and invalid. You need to pull values from the database row and assign them to the Session, like $_SESSION['fullname'] = $row['fullname']; (adjust the column name to match your actual database field)
• Fix error handling for queries: Use mysqli_error($db) instead of mysqli_connect_error() when querying fails—connect_error is only for database connection issues
• Fix SQL Injection Risk: Never directly insert user input like $uname into a query. Use mysqli_real_escape_string() (quick fix) or prepared statements (more secure long-term)

Corrected Full Code #

<?php
// Start the session first (required for Sessions to work across pages)
session_start();

$db = mysqli_connect("localhost", "joy", "hvkgigkgijhkhi");
if(!$db){ 
    echo "No DB Connection"; 
} else {
    // Fixed: Correct mysqli_select_db parameter order
    mysqli_select_db($db, "testing");

    // Fixed: Escape user input to block SQL injection
    $safe_uname = mysqli_real_escape_string($db, $uname);
    $query = "SELECT * FROM member WHERE uname= '$safe_uname'";

    // Fixed: Pass database connection to mysqli_query
    $results = mysqli_query($db, $query);

    if(!$results){ 
        // Fixed: Use query-specific error message
        die('Could not run query: ' . mysqli_error($db)); 
    } else {
        // Fetch the matching user row (no need for a loop if uname is unique)
        $row = mysqli_fetch_array($results, MYSQLI_ASSOC);
        if($row) { // Only assign Sessions if a user was found
            // Fixed: Properly assign database values to Session variables
            $_SESSION['uname'] = $row['uname'];
            $_SESSION['fullname'] = $row['Fname']; // Adjust column name if your DB uses something else
            $_SESSION['my_ref_id'] = $row['My_ref'];
            $_SESSION['email'] = $row['email'];
            // $_SESSION['success'] = "You are now logged in";
            // header('location: index.php');
        }
    }
}
?>

<!-- logged in user information -->
<div class="menue">
<?php if (isset($_SESSION['uname'])) : ?>
    <p>Refferal Pin: <strong><?php echo $_SESSION['my_ref_id']; ?></strong><br></p>
    <p>Full Name: <strong><?php echo $_SESSION['fullname']; ?></strong><br></p>
    <p>User Name: <strong><?php echo $_SESSION['uname']; ?></strong><br></p>
    <p>Email Address: <strong><?php echo $_SESSION['email']; ?></strong><br></p>
    <p>
        <a href="index.php?logout='1'" style="color: red;">logout</a>
    </p>
<?php endif ?>
</div>

Quick Additional Notes #

• Ensure session_start(); is at the top of every page that uses Sessions—without it, your Session values won't persist
• Double-check that column names in $row['column_name'] match exactly what's in your member table (e.g., if your full name column is full_name instead of Fname, adjust that)
• For long-term security, consider switching to prepared statements (more robust against injection):

// Quick prepared statement example for your query
$stmt = mysqli_prepare($db, "SELECT * FROM member WHERE uname = ?");
mysqli_stmt_bind_param($stmt, "s", $uname);
mysqli_stmt_execute($stmt);
$results = mysqli_stmt_get_result($stmt);
$row = mysqli_fetch_array($results, MYSQLI_ASSOC);

内容的提问来源于stack exchange，提问作者Anthony Holland