Show source of a PHP file its .txt extension if it exists.

self-source.php

<?php

# Show source of this file in .txt extension if it exists
call_user_func(function() {
    $txt = __FILE__ . '.txt';
    if ( ! file_exists($txt))
        return;
    # update (cache at least 5-min)
    if (300 < time() - filemtime($txt))
        file_put_contents($txt, file_get_contents(__FILE__));
    # message
    $txt = basename($txt);
    echo "/* PHP source: $txt */\n";
});