Verified on master.

you can not destroy a closure while you are calling it.

when you override $f in $f, zend vm try to destroy the closure $f, since the 
refcout of it is 1.

try following :

$b = $f = function() use (&$f) {
    $f = function() {};
};
$f();
$f();

hi @laurence, thank's for taking the time to review it. If I write code like

unset($this);

it does not fatal error either (you can not destroy a object while you are calling 
it.).

Also I don't want to destroy the closure, I just want to re-use that variable. 
Can't you just let the garbage collector do the dirty work?

@laruence: The closure can still exist even if it is not referenced by $f anymore. I didn't look into this yet, but the fix should be along the lines of adding a ref to the closure when it is called and removing it again when it finishes running. Actually, I remember seeing something in the code that backs up the function zval into op_array.prototype (disguised as a zend_function*) and dtors it in the leave_helper. But clearly that isn't enough yet.

I just looked into this a bit but couldn't find a good way to fix it. The main issue is that the prototype hack is only done in ZEND_INIT_FCALL_BY_NAME and only for VARs. Doing the same for CVs (as in this case) would be easy, but it sadly does not work properly if the closure is also invoked using call_user_func or any other function using zend_call_function internally. zend_call_function does not back the closure into the prototype and due to the way it works I'm not sure how this could be added there. For zcf the get_closure object handler call is done within zend_is_callable_ex and the results are put into the passed function_call_cache then. But the fcc can be reused for multiple calls (or not be used at all), so I really don't know how to do this safely without causing leaks or double frees.

This bug is really annoying. Here's another scenario that I just hit while refactoring:

set_exception_handler(function($exception) {
    restore_exception_handler(); // boom
    echo "Exception handled, move along.\n";
});

throw new Exception; // fatal error: cannot destroy active lambda

@john dot papaioannou: If it helps, you can prevent the error in your case by assigning the closure to a variable first, so that the exception handler can be reset without destroying the closure (here: $handler):

set_exception_handler($handler = function($exception) {
    restore_exception_handler(); // boom
    echo "Exception handled, move along.\n";
});

throw new Exception; // fatal error gone now

Demo: http://3v4l.org/bJv3p

I know that this does not fix the underlying problem, just commenting in the hope this might be helpful for you.

This bug has been fixed with http://git.php.net/?p=php-src.git;a=commitdiff;h=ed9c8a23ba573b3e81399a158f3dee4e9708b6ad in PHP 7.