How can PHP be used to prevent the creation of usernames with multiple passwords during registration?

To prevent the creation of usernames with multiple passwords during registration, we can implement a check in the PHP code that compares the entered password with any previously registered passwords for that username. If a match is found, the registration process can be halted and an error message can be displayed to the user.

// Check if the entered password matches any previously registered passwords for the username
if ($enteredPassword == $storedPassword) {
    // Display an error message and halt the registration process
    echo "Error: Password already in use for this username.";
    exit;
}