I've figured out how to work around this (by saving the assertion state in the assert handler and letting the error handling continue to the error handler), so I've decided to close this.

Could you post the way you did?

Here's a basic example:

class ErrorHandler {    
    private $assertion;

    public static function enable() {        
        // Configure errors.
        error_reporting(E_ALL);        
        
        // Configure assertions.
        assert_options(ASSERT_ACTIVE, 1);
        assert_options(ASSERT_WARNING, 1);
        assert_options(ASSERT_BAIL, 0);
        assert_options(ASSERT_QUIET_EVAL, 0);

        // Register handlers.
        $eh = new ErrorHandler();
        set_error_handler([$eh, 'handle_error'], error_reporting());
        assert_options(ASSERT_CALLBACK, [$eh, 'handle_assertion']);
    }
    
    public function handle_assertion($file, $line, $code, $desc = null) {
        // Save the raw assertion code and description.
        // The actual assertion is handled in the error handler.
        $this->assertion = [$code, $desc];
    }

    public function handle_error($errno, $errstr, $errfile, $errline, $errcontext) {
        if (!(error_reporting() & $errno)) {
            // This error code is not included in error_reporting.
            return;
        }

        if (!$this->assertion) {
            throw new ErrorException($errstr, 0, $errno, $errfile, $errline);
        }

        list($code, $desc) = $this->assertion;
        $this->assertion = null; // Reset the assertion.
        // Handle the assertion here.
    }
}