Resource Types
Hash Functions
Hash
PHP Manual

# 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.

Resource Types
Hash Functions
Hash
PHP Manual