Hidef is a read only simple cache intended to reduce data copy
overhead that other methods have.

=Why a separate repo when one exists at https://pecl.php.net/package/hidef ?

Original hidef extension does not support php 5.6.* versions. I tried
asking for PECL account and push changes to original repo, but PHP folks
never approved my account. So code in this repo is same as original repo
its just that I have added support for PHP 5.6.* versions. Feel free to file
issue if there problem related to any specific version.

=Why write this?

This extension was originally meant as a replacement for 
apc_load_constants(). It was meant to replace constants in
php scripts with in-place compile-time constants. Eventually,
everything in APC that could be done read-only got shunted 
into it.

=How does it work?

Hidef is initialized in php module init, before apache starts spawning
children. Hidef loads up all files "*.data" in a given hidef.data_dir
ini param, unserializes them and copies them into persistent php memory.

Since the engine init is not done at this point, unserializing an
object would crash the system. So the data can only be basic php
datastructures - arrays, hashes & scalars.

After it is loaded, this data is not modified. But PHP uses refcount
to track object references, so giving out persistent data to php 
function calls is not possible. APC uses a full-copy approach which
copies the entire array/hash into local memory before returning. This
is expensive, but APC has to do it to avoid locking the cache for the
entire access pattern. 

Hidef doesn't have to lock, because the data is immutable. And the pointers
are valid forever in that process. For scalar data types, the cost of 
copying is minimal, but for arrays & hashes, we can wrap them in 
an accesor object pointing to the data in immutable space. Since the
objects are opaque constructs, the refcounts of the objects do not 
affect the refcount of the arrays & consistency is maintained.

FrozenArray is the name of this data structure. This allows array like
access with $a["x"] and iteration using foreach() loops.

If the original data structure is required for some reason, like a
array_keys() call later on, this can be thawed into a the original
data structure by doing a $a->thaw() or an even simpler (array)($a)
cast. A thaw operation is almost identical to the unserialize operation,
except since the data is already in memory, the operation is faster.

=Where to use it for 

Something like a localization table would be ideal to store in 
this cache. Since no updates are possible in hidef without restarting
apache, data with a short life-span obviously doesn't belong in this.