register_shutdown_function

function my_shutdown() {
    // Whatever cleanup routines, DB, logging, etc.
}

function nice_exit() {
    // Call the register shutdown function
    my_shutdown();
    // Hard Exit
    exit;
}

register_shutdown_function("my_shutdown");

// code to execute
if ($some_condition) {
    nice_exit();
}
// code that might not execute