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function is not described

_hash_hmac() WP 3.2.0

Internal compat function to mimic hash_hmac().

No Hooks.

Return

String/false. The hash in output determined by $raw_output. False if $algo is unknown or invalid.

Usage

_hash_hmac( $algo, $data, $key, $raw_output );
$algo(string) (required)
Hash algorithm. Accepts 'md5' or 'sha1'.
$data(string) (required)
Data to be hashed.
$key(string) (required)
Secret key to use for generating the hash.
$raw_output(true/false)
Whether to output raw binary data (true), or lowercase hexits (false).
Default: false

Code of hash hmac: wp-includes/compat.php VER 5.0.3

<?php
function _hash_hmac($algo, $data, $key, $raw_output = false) {
	$packs = array('md5' => 'H32', 'sha1' => 'H40');

	if ( !isset($packs[$algo]) )
		return false;

	$pack = $packs[$algo];

	if (strlen($key) > 64)
		$key = pack($pack, $algo($key));

	$key = str_pad($key, 64, chr(0));

	$ipad = (substr($key, 0, 64) ^ str_repeat(chr(0x36), 64));
	$opad = (substr($key, 0, 64) ^ str_repeat(chr(0x5C), 64));

	$hmac = $algo($opad . pack($pack, $algo($ipad . $data)));

	if ( $raw_output )
		return pack( $pack, $hmac );
	return $hmac;
}