What is the purpose of the isset() function in PHP?

The isset() function in PHP is designed to check whether a variable has been declared and is not NULL. This function returns true if the variable exists and holds a value that is not equal to NULL, making it a fundamental tool for verifying variable status before performing operations that rely on their presence.

Using isset() is particularly useful in scenarios where you want to prevent undefined variable errors or ensure that necessary input values are provided before executing business logic. For example, when handling form submissions, developers often use isset() to determine if certain expected inputs have been sent by the user, thereby allowing them to proceed with processing the data only when valid and set values are present.

Other options do not accurately describe the purpose of isset(). The function does not determine if a variable is considered empty—that's handled by the empty() function. Similarly, isset() does not validate a variable's type, as type validation is performed using functions like is_int(), is_string(), and others. Finally, the destruction of session variables is managed using session functions like unset() or session_destroy(), rather than with isset().