Symfony 2.3.16
PHP 5.4.41-0+deb7u1 (cli) (built: May 28 2015 18:22:26)

https://github.com/symfony/symfony/blob/master/src/Symfony/Component/HttpKernel/HttpCache/ResponseCacheStrategy.php#L79.

In the case of a page /example-1

• Page (s-max-age: 86400)
  • block A (s-max-age: null)
  • block B (s-max-age: 0)

The s-max-age injected by symfony in the response is not 0 but 86400.

Validation by test : php -r "var_dump(min(array(1,2,3,null, 0)));"

In the case of a page /example-2

• Page (s-max-age: 86400)
  • block A (s-max-age: 0)
  • block B (s-max-age: null)

The s-max-age injected by symfony in the response is 0.

Validation by test : php -r "var_dump(min(array(1,2,3, 0, null)));"