The problem seems to be that usort checks the amount of references 
before and after the function call to see if the user-provided function 
modified it, but inside the function call, debug_backtrace_get_args adds 
a reference to the passed variables to use in e.g. debug_backtrace's 
"arg" element.

This was caused by the fix for bug #50006 (there weren't such checks before :)

Stas, can you check this out? Didn't expect anyone to use exceptions, did you? :D

affects gentoo builds after > 5.2.10 (5.2.11, 5.2.11-r1, and 5.2.12)

The reason seems to be that when making exception backtrace, debug_backtrace_get_args() uses SEPARATE_ZVAL_TO_MAKE_IS_REF() on arguments, which makes it look as if the argument was indeed modified (which usort is designed to protect against, since cmp callback is not supposed to modify the arguments)

I printed a debug line from my usort callback. It called debug_backtrace() to print the line and sourcefile in the debuglog. And therefor triggered the error. Even more, it did not sort.

Maybe the phpmanual should state that usort() callbacks are not allowed to write loglines. I also think that usort() callbacks that DO change the array are perfectly legal, as long as they don't change the sort.  

Maybe your sorter code needs stackoverflow protection or whatever, but calling certain code 'invalid', because it causes your code to SEGV is a stupid way to solve a bug.

I notice this is still affecting PHP 5.3.3 (Windows/Apache install).

Is this likely to be fixed soon - is it a question of developer time and priority 
or is it too difficult to fix?

It's quite irritating - I realise that the obvious solution is to avoid throwing 
the exception (ha-ha) but it's a useful function and exceptions are... inevitable.

This bug is still present as of PHP 5.3.8, we ran into it today and spent most of a day trying to figure out what was causing the error message "Array was modified by the user comparison function", when CLEARLY, NOTHING was changing the array at all!

The exception was not thrown/caught directly in the usort function but rather in a constructor of a class that was called about 3 or 4 functions deep from the usort, making it very difficult to track down.  

After finally figuring out the exception was somehow related, we searched google and found this bug report.  I'm sure we can agree that the minor act of catching an exception should not result in usort throwing a warning message.  This bug is a huge timewaster :(

It took me a while to figure out that some code called from usort was throwing, catching, and (gracefully) handling an Exception.  Then I found this post.  Quite frustrating.

I turned off warnings with ini_set before calling usort, then turned them on again after.  This is an effective workaround for now, but I'd love to clean that nastiness out of my code.

It is also my opinion that usort should be allowed to change the elements in the array.  EG: an instance variable of an object may be lazy-loaded as a result of a method call from within a usort callback.  Should a warning really be issued in that case?

This same problem arises when using Mockery to mock the object whose method is 
being used by usort(), even though the method itself neither is mocked nor handles 
any exceptions. The proxy generated by Mockery must wrap the target class's 
methods with some exception-handling code.

Unfortunately this forced me to code a workaround that would not use usort. My 
hack extracts from the objects in the array the values being sorted on, sorts that 
array of values using asort() (to preserve the keys), and finally rebuilds the 
list of objects using the keys in the order that they appear in the asorted list 
of values. Yuck.

This will probably be obvious to most, but I just wanted to mention that you can always prefix the usort function with the @ symbol to prevent the warning...of course that would also suppress any other types of notices or warnings that might occcur anywhere within the sorting function...

Php 5.4.16 also fails with this.
Still the same status for 3 and a half years old bug?!

I'd like to add, that you do not have to throw an exception to get this warning.
Mere creating it, also triggers the warning, as in:

<?php
function comp($a,$b){
  @new Exception("dupa");
}
$a =array(1,2,3);
  usort($a, 'comp');
var_dump($a);
?>

PHP Warning:  usort(): Array was modified by the user comparison function in 
/home/jlopuszanski/test.php on line 6

I ran into a similar issue, i'm sure it'll require the same patch as it's the 
backtrace causing the problem but worth noting it doesn't require an exception to 
trigger this, just a backtrace.

$ cat usort.php
<?php
 
set_error_handler(function($errno, $errstr) {
  $bt = debug_backtrace();
  var_dump($errstr);
});
 
$arr = [1, 2];
usort($arr, function($a, $b) use ($arr) {
  trigger_error('test');
  return $a > $b;
});
 
$ php usort.php
string(4) "test"
string(59) "usort(): Array was modified by the user comparison function"

Sorry the use () isn't relevant, I forgot to remove it when simplifying my test 
case

This has been fixed in PHP 7.

A notice within the compare function will also trigger this notice. For example:

function test($a, $b) {
  return strnatcmp($a['name'], $b['name']);
}
$e = array(
  array(
    'not-name' => 'a',
  ),
  array(
    'name' => '',
  ),
);
uasort($e, 'test');

(this is tested on PHP 5.3)

This bug should be closed as Fixed, since it's fixed in PHP 7 and 5.6 does not have active support.

Closing as this is fixed in PHP 7 and PHP 5.6 is out of active support.