以下常量由本扩展模块定义，因此只有在本扩展模块被编译到 PHP 中，或者在运行时被动态加载后才有效。
Enables or disables payload compression. When enabled, item values longer than a certain threshold (currently 100 bytes) will be compressed during storage and decompressed during retrieval transparently.
Type: boolean, default: TRUE.
Specifies the serializer to use for serializing non-scalar values. The valid serializers are Memcached::SERIALIZER_PHP or Memcached::SERIALIZER_IGBINARY. The latter is supported only when memcached is configured with --enable-memcached-igbinary option and the igbinary extension is loaded.
Type: integer, default: Memcached::SERIALIZER_PHP.
The default PHP serializer.
The » igbinary serializer. Instead of textual representation it stores PHP data structures in a compact binary form, resulting in space and time gains.
This can be used to create a "domain" for your item keys. The value specified here will be prefixed to each of the keys. It cannot be longer than 128 characters and will reduce the maximum available key size. The prefix is applied only to the item keys, not to the server keys.
Type: string, default: "".
Specifies the hashing algorithm used for the item keys. The valid values are supplied via Memcached::HASH_* constants. Each hash algorithm has its advantages and its disadvantages. Go with the default if you don't know or don't care.
Type: integer, default: Memcached::HASH_DEFAULT
The default (Jenkins one-at-a-time) item key hashing algorithm.
MD5 item key hashing algorithm.
CRC item key hashing algorithm.
FNV1_64 item key hashing algorithm.
FNV1_64A item key hashing algorithm.
FNV1_32 item key hashing algorithm.
FNV1_32A item key hashing algorithm.
Hsieh item key hashing algorithm.
Murmur item key hashing algorithm.
Specifies the method of distributing item keys to the servers. Currently supported methods are modulo and consistent hashing. Consistent hashing delivers better distribution and allows servers to be added to the cluster with minimal cache losses.
Type: integer, default: Memcached::DISTRIBUTION_MODULA.
Modulo-based key distribution algorithm.
Consistent hashing key distribution algorithm (based on libketama).
Enables or disables compatibility with libketama-like behavior. When enabled, the item key hashing algorithm is set to MD5 and distribution is set to be weighted consistent hashing distribution. This is useful because other libketama-based clients (Python, Ruby, etc.) with the same server configuration will be able to access the keys transparently.
Note: It is highly recommended to enable this option if you want to use consistent hashing, and it may be enabled by default in future releases.
Type: boolean, default: FALSE.
Enables or disables buffered I/O. Enabling buffered I/O causes storage commands to "buffer" instead of being sent. Any action that retrieves data causes this buffer to be sent to the remote connection. Quitting the connection or closing down the connection will also cause the buffered data to be pushed to the remote connection.
Type: boolean, default: FALSE.
Enable the use of the binary protocol. Please note that you cannot toggle this option on an open connection.
Type: boolean, default: FALSE.
Enables or disables asynchronous I/O. This is the fastest transport available for storage functions.
Type: boolean, default: FALSE.
Enables or disables the no-delay feature for connecting sockets (may be faster in some environments).
Type: boolean, default: FALSE.
The maximum socket send buffer in bytes.
Type: integer, default: varies by platform/kernel configuration.
The maximum socket receive buffer in bytes.
Type: integer, default: varies by platform/kernel configuration.
In non-blocking mode this set the value of the timeout during socket connection, in milliseconds.
Type: integer, default: 1000.
The amount of time, in seconds, to wait until retrying a failed connection attempt.
Type: integer, default: 0.
Socket sending timeout, in microseconds. In cases where you cannot use non-blocking I/O this will allow you to still have timeouts on the sending of data.
Type: integer, default: 0.
Socket reading timeout, in microseconds. In cases where you cannot use non-blocking I/O this will allow you to still have timeouts on the reading of data.
Type: integer, default: 0.
Timeout for connection polling, in milliseconds.
Type: integer, default: 1000.
Enables or disables caching of DNS lookups.
Type: boolean, default: FALSE.
Specifies the failure limit for server connection attempts. The server will be removed after this many continuous connection failures.
Type: integer, default: 0.
The operation was successful.
The operation failed in some fashion.
DNS lookup failed.
Failed to read network data.
Bad command in memcached protocol.
Error on the client side.
Error on the server side.
Failed to write network data.
Failed to do compare-and-swap: item you are trying to store has been modified since you last fetched it.
Item was not stored: but not because of an error. This normally means that either the condition for an "add" or a "replace" command wasn't met, or that the item is in a delete queue.
Item with this key was not found (with "get" operation or "cas" operations).
Partial network data read error.
Some errors occurred during multi-get.
Server list is empty.
End of result set.
System error.
The operation was buffered.
The operation timed out.
Bad key.
Failed to create network socket.
Payload failre: could not compress/decompress or serialize/unserialize the value.