III. Alternative PHP Cache


The Alternative PHP Cache (APC) is a free and open opcode cache for PHP. It was conceived of to provide a free, open, and robust framework for caching and optimizing PHP intermediate code.


This PECL extension is not bundled with PHP.

Information for installing this PECL extension may be found in the manual chapter titled Installation of PECL extensions. Additional information such as new releases, downloads, source files, maintainer information, and a CHANGELOG, can be located here: http://pecl.php.net/package/apc.

You may download this PECL extension DLL from the PHP Downloads page or at http://snaps.php.net/.

Note: On Windows, APC needs a temp path to exist, and be writable by the web server. It checks TMP, TEMP, USERPROFILE environment variables in that order and finally tries the WINDOWS directory if none of those are set.

Note: For more in-depth, highly technical implementation details, see the developer-supplied TECHNOTES file .

Runtime Configuration

The behaviour of these functions is affected by settings in php.ini.

Although the default APC settings are fine for many installations, serious users should consider tuning the following parameters.

There are two main decisions you have to make. First, how much shared memory do you want to set aside for APC, and second, whether you want APC to check if a file has been modified on every request. The two ini directives involved here are apc.shm_size and apc.stat. Read the sections on these two directives carefully below.

Once you have a running server, you should copy the apc.php script that comes with the extension to somewhere in your docroot and load it up in your browser. It provides you with a detailed look at what is happening in your cache. If you have GD enabled in PHP, it will even have pretty graphs. First thing to check is of course that it is actually caching files. Assuming it is working you should then pay close attention to the Cache full count number on the left. That tells you the number of times the cache has filled up and has had to forcefully clean up any entries not accessed within the last apc.ttl seconds. You should configure your cache to minimize this number. If you are constantly filling your cache, the resulting cache churn is going to hurt performance. You should either set more memory aside for APC, or use apc.filters to cache fewer scripts.

Table 1. APC configuration options

apc.enable_cli"0"PHP_INI_SYSTEM> APC 3.0.6
apc.max_file_size"1M"PHP_INI_SYSTEM> APC 3.0.6
apc.stat"1"PHP_INI_SYSTEM> APC 3.0.9
For further details and definitions of the PHP_INI_* constants, see the Appendix G.

Here's a short explanation of the configuration directives.

apc.enabled boolean

apc.enabled can be set to 0 to disable APC. This is primarily useful when APC is statically compiled into PHP, since there is no other way to disable it (when compiled as a DSO, the extension line in php.ini can just be commented-out).

apc.shm_segments integer

The number of shared memory segments to allocate for the compiler cache. If APC is running out of shared memory but you have already set apc.shm_size as high as your system allows, you can try raising this value.

apc.shm_size integer

The size of each shared memory segment in MB. By default, some systems (including most BSD variants) have very low limits on the size of a shared memory segment.

apc.optimization integer

The optimization level. Zero disables the optimizer, and higher values use more aggressive optimizations. Expect very modest speed improvements. This is experimental.

apc.num_files_hint integer

A "hint" about the number of distinct source files that will be included or requested on your web server. Set to zero or omit if you're not sure; this setting is mainly useful for sites that have many thousands of source files.

apc.ttl integer

The number of seconds a cache entry is allowed to idle in a slot in case this cache entry slot is needed by another entry. Leaving this at zero means that your cache could potentially fill up with stale entries while newer entries won't be cached.

apc.gc_ttl integer

The number of seconds that a cache entry may remain on the garbage-collection list. This value provides a fail-safe in the event that a server process dies while executing a cached source file; if that source file is modified, the memory allocated for the old version will not be reclaimed until this TTL reached. Set to zero to disable this feature.

apc.cache_by_default boolean

On by default, but can be set to off and used in conjunction with positive apc.filters so that files are only cached if matched by a positive filter.

apc.filters string

A comma-separated list of POSIX extended regular expressions. If any pattern matches the source filename, the file will not be cached. Note that the filename used for matching is the one passed to include/require, not the absolute path. If the first character of the expression is a + then the expression will be additive in the sense that any files matched by the expression will be cached, and if the first character is a - then anything matched will not be cached. The - case is the default, so it can be left off.

apc.mmap_file_mask string

If compiled with MMAP support by using --enable-mmap this is the mktemp-style file_mask to pass to the mmap module for determing whether your mmap'ed memory region is going to be file-backed or shared memory backed. For straight file-backed mmap, set it to something like /tmp/apc.XXXXXX (exactly 6 Xs). To use POSIX-style shm_open/mmap put a .shm somewhere in your mask. e.g. /apc.shm.XXXXXX You can also set it to /dev/zero to use your kernel's /dev/zero interface to anonymous mmap'ed memory. Leaving it undefined will force an anonymous mmap.

apc.slam_defense integer

On very busy servers whenever you start the server or modify files you can create a race of many processes all trying to cache the same file at the same time. This option sets the percentage of processes that will skip trying to cache an uncached file. Or think of it as the probability of a single process to skip caching. For example, setting apc.slam_defense to 75 would mean that there is a 75% chance that the process will not cache an uncached file. So, the higher the setting the greater the defense against cache slams. Setting this to 0 disables this feature.

apc.file_update_protection integer

When you modify a file on a live web server you really should do so in an atomic manner. That is, write to a temporary file and rename (mv) the file into its permanent position when it is ready. Many text editors, cp, tar and other such programs don't do this. This means that there is a chance that a file is accessed (and cached) while it is still being written to. This apc.file_update_protection setting puts a delay on caching brand new files. The default is 2 seconds which means that if the modification timestamp (mtime) on a file shows that it is less than 2 seconds old when it is accessed, it will not be cached. The unfortunate person who accessed this half-written file will still see weirdness, but at least it won't persist. If you are certain you always atomically update your files by using something like rsync which does this correctly, you can turn this protection off by setting it to 0. If you have a system that is flooded with io causing some update procedure to take longer than 2 seconds, you may want to increase this a bit.

apc.enable_cli integer

Mostly for testing and debugging. Setting this enables APC for the CLI version of PHP. Normally you wouldn't want to create, populate and tear down the APC cache on every CLI request, but for various test scenarios it is handy to be able to enable APC for the CLI version of APC easily.

apc.max_file_size integer

Prevent files larger than this value from getting cached. Defaults to 1M.

apc.stat integer

Be careful if you change this setting. The default is for this to be On which means that APC will stat (check) the script on each request to see if it has been modified. If it has been modified it will recompile and cache the new version. If you turn this setting off, it will not check. That means that in order to have changes become active you need to restart your web server. On a production server where you rarely change the code, turning stats off can produce a significant performance boost.

For included/required files this option applies as well, but note that if you are using relative path includes (any path that doesn't start with / on Unix) APC has to check in order to uniquely identify the file. If you use absolute path includes APC can skip the stat and use that absolute path as the unique identifier for the file.

Resource Types

This extension has no resource types defined.

Predefined Constants

This extension has no constants defined.

Table of Contents
apc_cache_info --  Retrieves cached information (and meta-data) from APC's data store
apc_clear_cache --  Clears the APC cache
apc_define_constants --  Defines a set of constants for later retrieval and mass-definition
apc_delete --  Removes a stored variable from the cache
apc_fetch --  Fetch a stored variable from the cache
apc_load_constants --  Loads a set of constants from the cache
apc_sma_info --  Retrieves APC's Shared Memory Allocation information
apc_store --  Cache a variable in the data store