PHP RFC: Catchable "call to a member function of a non-object"

• Version: 1.1
• Date: 2014-04-26
• Author: Timm Friebe thekid@php.net
• Status: Accepted
• First Published at: https://wiki.php.net/rfc/catchable-call-to-member-of-non-object

One of the most common fatal errors in PHP is the “call to a member function of a non-object” type. This occurs whenever a method is called on anything other than an object (usually null), e.g.:

// ...when getAction() returns null:
$this->getAction()->invoke();

One situation in which fatal errors are problematic is if you want to run PHP as a webserver itself. For a long story on why you would want to do that in the first place, see http://marcjschmidt.de/blog/2014/02/08/php-high-performance.html.

Other situtations are described in the Engine Exceptions RFC.

This proposal's essence is to turns fatal errors generated from calls to methods on a non-object into E_RECOVERABLE_ERRORs.

set_error_handler(function($code, $message) {
  var_dump($code, $message);
});
 
$x= null;
var_dump($x->method());
echo "Alive\n";

The above produces the following output:

int(4096)
string(50) "Call to a member function method() on a non-object"
NULL
Alive

This behavior is consistent with how type hints work. Framework authors can turn this into exceptions if they wish.

The following error handler could be embedded into frameworks:

set_error_handler(function($code, $message) {
  if (0 === strncmp('Call', $message, 4)) {
    throw new BadMethodCallException($message);
  } else if (0 === strncmp('Argument', $message, 8)) {
    throw new InvalidArgumentException($message);
  } else {
    trigger_error($message, E_USER_ERROR);
  }
}, E_RECOVERABLE_ERROR);
 
$x= null;
try {
  $x->method();
} catch (BadMethodCallException $e) {
  echo "Caught expected ", $e->getMessage(), "!\n";
}
echo "Alive\n";

This could be a way for people preferring not to use exceptions and instead to exit the script directly, but get a stacktrace instead of just the fatal error message:

set_error_handler(function($code, $message) {
  echo "*** Error #$code: $message\n";
  debug_print_backtrace();
  exit(0xFF);
}, E_RECOVERABLE_ERROR);
 
$m= new some_db_model();
$row= $m->find(42); // null, deleted concurrently
$row->delete();

This proposal doesn't go as far as the controversial RFC RFC: Engine exceptions.

Taken this code:

function a($comparator) {
  $result= $comparator->compare(1, 2);
  // ...
}

You can unwind this to something like:

function a($comparator) {
  1: ZEND_INIT_METHOD_CALL $comparator 'compare'
  2: ZEND_SEND_VAL         1
  3: ZEND_SEND_VAL         2
  4: ZEND_DO_FCALL_BY_NAME
  5: ZEND_ASSIGN           $result
  // ...
}

The handling on checking whether the method is callable happends inside the opline #1 (ZEND_INIT_METHOD_CALL). The opcode handler for it checks its first argument (here: $comparator) whether it is an object or not. In case it's not, the following happens:

1. A recoverable error is triggered. Should the script exit here, there's nothing more to be done, this is just as it was before.
2. In case the script continues, all the oplines are skipped until we find the FCALL_BY_NAME opcode. This is comparable to jumping forward just as, e.g., the ZEND_JMP instruction would.
3. The return value is set to ZVAL_NULL().
4. The control is handed back to the executor, which then continues with the ASSIGN opcode
5. The engine is again in full control; if an exception was raised by the handler, that leads to the known behavior.

This RFC is backwards compatible with previous PHP releases.

There is no impact on any SAPI.

No impact.

No effect, before, the script terminated.

This RFC targets PHP 5.7 or PHP 6.0, whichever comes first.

This RFC modifies the PHP language behaviour and therefore requires a two-third majority of votes.

There is a pull request available over at GitHub which includes tests. Feedback welcome!

Ideas for future work include:

• Also allowing to catch and handle other fatal errors

Voting started 2014-06-29 and ended 2014-07-30.

Catchable Call to a member function bar() on a non-object
Real name	Yes	No
ajf (ajf)
brandon (brandon)
brianlmoon (brianlmoon)
bwoebi (bwoebi)
crodas (crodas)
datibbaw (datibbaw)
davey (davey)
dragoonis (dragoonis)
fa (fa)
frozenfire (frozenfire)
gooh (gooh)
guilhermeblanco (guilhermeblanco)
gwynne (gwynne)
indeyets (indeyets)
jedibc (jedibc)
jpauli (jpauli)
jwage (jwage)
kassner (kassner)
kriscraig (kriscraig)
levim (levim)
malukenho (malukenho)
mrook (mrook)
nikic (nikic)
oaass (oaass)
philstu (philstu)
rdlowrey (rdlowrey)
rdohms (rdohms)
reeze (reeze)
seld (seld)
treffynnon (treffynnon)
tyrael (tyrael)
zhangzhenyu (zhangzhenyu)
Final result:	32	0
This poll has been closed.

• PHP Bugs 46601, 51882 and 51848- bugs which would be fixed by this
• PHP Bug 54195 - related, motivates necessity
• HHVM throws a BadMethodCallException in these situations
• Mailing list announcement
• Return type hinting RFC - related work: Some fatal errors can be prevented by setting return types.