What role does the $sound_type variable play in the PHP code provided, and how should it be defined for proper functionality?

The $sound_type variable in the PHP code provided is used to determine the type of sound that will be played. To ensure proper functionality, the $sound_type variable should be defined with a valid sound type such as "beep" or "alarm" before being used in the switch statement to play the corresponding sound.

$sound_type = "beep"; // Define the sound type here

switch ($sound_type) {
    case "beep":
        echo "Playing beep sound";
        // Code to play beep sound
        break;
    case "alarm":
        echo "Playing alarm sound";
        // Code to play alarm sound
        break;
    default:
        echo "Invalid sound type";
}