Description:
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shm provides more speed than files or databases. But there is no
proper way to check if a field exists. You can do
@shm_get_var($connection, $field) !== false to check it. But:
-With custom error handlers you will get a warning, when there is no
such $field
-It needs a lot of time to process the warning, but it should be fast

There should be a function shm_has_var or shm_var_exists.

A function shm_export_vars could convert the shared memory to an
associative array.
A function shm_export_keys: Like
array_keys(shm_export_vars($connection)) but faster
Those functions are very essential. When you don't have them you have
to use the shared-memory as a linked list.
A function shm_get_size: Return the size of the shared memory
segment. Not necessary, because you can store it using
shm_put_var($connection, 0, $size), but such a function would be more
secure
Maybe also shm_resize could be possible.

With this function there could be better object oriented array like
wrappers form shared memory.