Fix caching of PHP_SAPI value in opcache file_cache

[YuriyNasretdinov]

When using opcache.file_cache option both in web and cli contexts you get very strange results when PHP_SAPI constant value has different values in different files (it depends on context in which file was included first: cli or web).

This pull requests makes attempt to fix this in a bit hacky way (we only disallow a single constant, PHP_SAPI, from being substituted).

I am not sure whether or not adding new constant like CONST_CT_NO_PERSISTENT_SUBST would be better.

[nikic]

Another option would be to include the sapi in the system ID, which would prevent the cache from being shared between different SAPIs. @dstogov What do you think about this? Is the cache supposed to be shared between SAPIs?

[YuriyNasretdinov]

I personally would like to be able to reuse file cache between CLI and Web because it means that it would consume 2 times less disk space. Also it allows to generate file cache once and deploy it. It is much more convenient in CLI mode rather than doing it from web server :).

[dstogov]

The patch prevents caching of PHP_SAPI not only in file_cache, but also in SHM.
Disabling all persistent constants substitution, using ZEND_COMPILE_NO_PERSISTENT_CONSTANT_SUBSTITUTION, may prevent substitution of other constants.

We may include SAPI into system ID, like @nikic proposed, or introduce a new constant/compilation flags that prevent compile-time constant substitution if file-cache is enabled.

[YuriyNasretdinov]

As far as I can see, patch should not prevent other persistent constants from substitution because there already was logic for persistent constant substitution in opcache extension, but execution never got to that branch because all persistent constants were already substituted.

I agree that this patch prevents caching of PHP_SAPI in SHM as well but I personally do not see it as a big problem because consistency is probably more important :).

Maybe it is not a bad idea to prevent all (persistent) constants from substitution but I am not sure how it is going to affect performance in CLI mode (performance boost is the primary reason for using file_cache in the first place in my opinion).

[dstogov]

I've committed another fix (into master only).

83f98f7