PHP RFC: Error backtraces


PHP errors, unlike exceptions, are much more limited in programmers' ability to control their handling and get detailed information about them. I propose to fix some of these issues by recording a backtrace for every error.

This problem is so serious that, for example, Wikimedia has created an extension for this.


Add backtraces to error output.

Error output: traces will be formatted exactly like for exceptions, and appended to error messages, e.g.:

Fatal error: Allowed memory size of 134217728 bytes exhausted in /var/www/test.php on line 123
#0 /var/www/test.php(123): spl_eat_all_memory()
#1 /var/www/foo.php(456): foo(123, 'foo')
#2 /var/www/bar.php(789): bar()
#3 {main}

Logging: backtraces will be logged just like for exceptions.

error_get_last() output will have another element added to it if a trace is available:

#1 {main}
array(5) {
  string(22) "Error, terrible error!"
  string(17) "/var/www/test.php"
  string(123) "#0 /var/www/test.php(345): $this->causeTerribleError()
#1 {main}"

Configuration: while always having backtraces is nice for debugging, some people might not like the performance overhead of generating backtraces or the risk of having to generate a trace after a fatal error. I therefore propose to address this with an INI setting, debug_backtraces that allows to turn this feature on and off, as well as to limit the number of stack frames outputted:

  • 0 - no backtraces
  • positive number - output this number of frames
  • negative value - no limit

By default it would be on in development settings but off in production.

Backward Incompatible Changes

  • In the current proposed implementation, a new error flag E_UNHANDLED_EXCEPTION is introduced to avoid outputting backtraces for unhandled exceptions twice, though it's not included in E_ALL and is invisible for userspace.
  • From the userspace POV, everything will behave exactly as before with backtraces disabled, while turning them on would introduce a few minor changes to how errors are represented.

Proposed PHP Version(s)


php.ini Defaults

For error_backtraces:

  • Default value: 0
  • php.ini-development value: 100
  • php.ini-production value: 0

Open Issues

Make sure there are no open issues when the vote starts!

Unaffected PHP Functionality

Everything other than the minor stuff mentioned in Backward Incompatible Changes.

Future Scope

Backtrace handling code could use some unification and refactoring, but that doesn't require a RFC and thus can be addressed later.

Proposed Voting Choices

  • Accept this RFC (yes/no)?

Patches and Tests


Links to external references, discussions or RFCs

Rejected Features

Keep this updated with features that were discussed on the mail lists.

rfc/error_backtraces.txt · Last modified: 2022/04/17 18:30 by ilutov