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# LIII. hash Functions

# Introduction

Message Digest (hash) engine. Allows direct or incremental processing of arbitrary length messages using a variety of hashing algorithms.

# Requirements

The Hash extension requires no external libraries and is enabled by default as of PHP 5.1.2. It may be explicitly disabled by using the --disable-hash switch to configure. Earlier versions of PHP may incorporate the Hash extension by installing the PECL module.

# Predefined Constants

The constants below are defined by this extension, and will only be available when the extension has either been compiled into PHP or dynamically loaded at runtime.

`HASH_HMAC` (integer)

  Optional flag for hash_init(). Indicates that the HMAC digest-keying algorithm should be applied to the current hashing context.

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