catch-exit.php

<?php

if (! function_exists('catch_exit')) {
    /**
     * Catch when a part of a script exits and execute a custom function at
     * that point. Note that if exiting, the script does not continue.
     * You can use this to, for example, flash a message and
     * redirect the user instead of showing the default
     * black text on white background "exit" view.
     *
     * @param Closure $run
     * @param mixed   $fallback
     * @return mixed
     */
    function catch_exit($run, $fallback)
    {
        $shouldExit = true;

        register_shutdown_function(function () use ($fallback, &$shouldExit) {
            if (! $shouldExit) {
                return;
            }

            $exitMessage = ob_get_contents();

            if ($exitMessage) {
                ob_end_clean();
            }

            if (is_object($fallback) && $fallback instanceof Closure) {
                $result = $fallback($exitMessage);
            } else {
                $result = $fallback;
            }

            echo $result;
        });
        
        ob_start();
        $result = $run();
        ob_end_clean();

        $shouldExit = false;

        return $result;
    }
}

if (! function_exists('catch_die')) {
    /**
     * Alias for catch_exit
     */
    function catch_die(...$args)
    {
        return catch_exit(...$args);
    }
}

Example usage:

$var = catch_exit(function() {
    return "It's all okay.";
}, "This is a custom exit message.");

echo $var; // "It's all okay."

$var = catch_exit(function() {
    exit;
    return "It's all okay.";
}, "This is a custom exit message.");

// The script exits before this point, displaying the vanilla 'exit' page
// with a white background and black text saying:
// "This is a custom exit message." 

echo $var;

$var = catch_exit(function() {
    return "It's all okay.";
}, function($message) {
    return "This is the original exit message: '{$message}'";
});

echo $var; // "It's all okay."

$var = catch_exit(function() {
    exit('Uh-oh, something went wrong!');
    return "It's all okay.";
}, function($message) {
    // Using Laravel 5's helper functions
    return back()->withInput()->with('error', "An error occurred: '{$message}'");
});

// The script exits before this point, however the redirect is ran and an error is
// flashed to the session, ready to be displayed to the user:
// "An error occurred: 'Uh-oh, something went wrong!'"

echo $var;