errorfunc: document the error handler stack - #5253
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set_error_handler() saves the handler that was active until then on an internal stack and installs the new one in its place; restore_error_handler() pops the saved one back, together with the error_levels mask it was registered with. Neither page said so, which invites restoring the previous handler by passing the return value of set_error_handler() back to it. That grows the stack instead of shrinking it, and resets error_levels to E_ALL. Adds a caution against that pattern and an example calling the previous handler from the new one. The seealso simplelists are unwrapped from their para, matching the exception handler pages.
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set_error_handler()saves the handler that was active until then on an internal stack and installs the new one in its place;restore_error_handler()pops it back, together with theerror_levelsmask it was registered with. Neither page said so, which invites passing the return value ofset_error_handler()back to it to "restore" it — that pushes a further entry instead of removing one, and silently resetserror_levelstoE_ALL.Both pages now describe the save/restore mechanism, the
nullcase and the empty-stack case. A caution warns against the re-registration pattern, and an example calls the previous handler from the new one. Theseealso<simplelist>are unwrapped from their<para>, matching the exception handler pages.The exception handler side is left out: it behaves differently since 8.3.5 and those pages already document it.
Sources:
ZEND_FUNCTION(set_error_handler)andZEND_FUNCTION(restore_error_handler)inZend/zend_builtin_functions.c;user_error_handleranduser_error_handlersinZend/zend_globals.h;shutdown_executor()inZend/zend_execute_API.c, which releases the stacks at the end of the request.Fixes: #4883