How can PHP prevent duplicate usernames during registration?

To prevent duplicate usernames during registration in PHP, you can query the database to check if the username already exists before allowing the user to register. If the username is already taken, display an error message prompting the user to choose a different username. 

// Check if the username already exists in the database
$username = $_POST['username'];
$query = "SELECT * FROM users WHERE username = '$username'";
$result = mysqli_query($connection, $query);

if(mysqli_num_rows($result) > 0) {
    // Username already exists, display an error message
    echo "Username already taken. Please choose a different username.";
} else {
    // Proceed with registration process
    // Insert user data into the database
}

Keywords

PHP registration duplicate usernames validation database.

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