What are the differences in functionality between using isset() and !isset() in PHP code, and how can they affect browser compatibility?

When using isset() in PHP, you are checking if a variable is set and not null. On the other hand, using !isset() checks if a variable is not set or is null. This can affect browser compatibility if the variable is not properly initialized or set before being used in the code.

// Using isset() to check if a variable is set and not null
if(isset($variable)){
    // Do something with the variable
}

// Using !isset() to check if a variable is not set or is null
if(!isset($variable)){
    // Handle the case where the variable is not set or is null
}