Are there specific PHP libraries or techniques, like runkit, that can enable dynamic function redefinition without the need for restarting the application?

One way to enable dynamic function redefinition in PHP without restarting the application is by using the `runkit` extension. This extension allows you to redefine functions at runtime, making it possible to update function definitions without restarting the application. Another approach is to use a combination of `eval()` and `create_function()` to redefine functions dynamically.

// Using runkit extension
runkit_function_redefine('myFunction', '$arg1, $arg2', 'return $arg1 + $arg2;');

// Using eval() and create_function()
$code = 'return $arg1 + $arg2;';
$dynamicFunction = create_function('$arg1, $arg2', $code);