In PHP, what are the differences between isset() and empty() functions when checking for the existence of variables with or without values?

When checking for the existence of variables with or without values in PHP, the isset() function is used to determine if a variable is set and is not NULL, while the empty() function is used to check if a variable is empty, which includes being set to an empty string, 0, NULL, or false. It's important to understand the differences between the two functions to accurately check the state of variables in your code.

// Using isset() to check if a variable is set and not NULL
if(isset($variable)){
    // Variable is set and not NULL
}

// Using empty() to check if a variable is empty
if(empty($variable)){
    // Variable is empty
}