index.php

<?php

function spl_push_error_handler(ErrorHandler $handler, int $level = \E_ALL, int $priority = 0): void
{
  // this function would be built into PHP, possibly with the addition of some extra functions to inspect and remove
  // existing error handlers
  
  // pushed error handlers would take priority over an error handlers set with set_errorhandler(), which would only be called
  // if the error is not handled by a handler in the stack. 
  
  // if neither a handler in the stack or the error handler set with set_errorhandler handles the error it performs the default
  // PHP action of dumping it to screen/logs
  
  // A polyfill for previous versions of PHP could be added in userland and set as the error handler with set_errorhandler,
  // however this suffers from the problem that it might get accidentilly removed by a user setting their own error handler
  // with that function. 
}

// Also defined as part of the language
interface ErrorHandler
{
    // should return true if the error has been handled by this handler, false otherwise to allow the next handler
    // in the stack to be invoked.
    public function handleError(int $errorNumber, string $errorMessage, ErrorContext $errorContext): bool;
}

// Also defined as part of the langage
class ErrorContext
{
    // file and line are returned as normal for the current error handling 
    public function getLine(): int
    public function getFile(): string
    // get class returns fully qualified class name in which the error occured, null if the error was not in a class
    public function getClass(): ?string
    // get function returns the fully qualified function name in which the error occurred, null if error was not in a function
    public function getFunction(): ?string
    // get method returns the method name the error occurred in, null if not in a method
    public function getMethod(): ?string
    // get backtrace returns an error backtrace as if the error were an exception
    public function getBacktrace(): array
}

class HandleDepreciations implements ErrorHandler
{
    public function handleError(int $errorNumber, string $errorMessage, ErrorContext $errorContext): bool
    {
        if (str_starts_with((string) $errorContext->getClass(), 'MY\\NAMESPACE')) {
            return true;
        }
      
        return false;
    }
}

//pushing error handlers could be simplified by a composer plugin if the library has no straight forward entry point 
spl_push_error_handler(new HandleDepreciations(), \E_DEPRECATED, 100);