How can the PHP code be modified to ensure that all variables are properly defined and initialized to prevent notices and errors during execution?

To ensure that all variables are properly defined and initialized to prevent notices and errors during execution, you can use the isset() function to check if a variable is set before using it. This way, you can avoid undefined variable notices and potential errors in your code.

// Check if variables are set before using them
if(isset($variable1, $variable2, $variable3)) {
    // Your code here
} else {
    // Handle the case where variables are not set
}