hash_hmac

Very important notice, if you pass array to $data, php will generate a Warning, return a NULL and continue your application. Which I think is critical vulnerability as this function used to check authorisation typically.
Example:
<?php
var_dump(hash_hmac('sha256', [], 'secret'));
WARNING hash_hmac() expects parameter 2 to be string, array given on line number 3
NULL
?>
Of course not documented feature.

As Michael uggests we should take care not to compare the hash using == (or ===). Since PHP version 5.6 we can now use hash_equals().
So the example will be:
<?php
 if (hash_equals($hashed_expected, $hashed_value) ) {
 echo "hashes match!";
 }
?>

Please be careful when comparing hashes. In certain cases, information can be leaked by using a timing attack. It takes advantage of the == operator only comparing until it finds a difference in the two strings. To prevent it, you have two options.
Option 1: hash both hashed strings first - this doesn't stop the timing difference, but it makes the information useless.
<?php
 if (md5($hashed_value) === md5($hashed_expected)) {
 echo "hashes match!";
 }
?>

Option 2: always compare the whole string.
<?php
 if (hash_compare($hashed_value, $hashed_expected)) {
 echo "hashes match!";
 }
 function hash_compare($a, $b) {
 if (!is_string($a) || !is_string($b)) {
 return false;
 }
 
 $len = strlen($a);
 if ($len !== strlen($b)) {
 return false;
 }
 $status = 0;
 for ($i = 0; $i < $len; $i++) {
 $status |= ord($a[$i]) ^ ord($b[$i]);
 }
 return $status === 0;
 }
?>

While implementing a TOTP application, please note that hash_hmac() must receive data in binary, not in a hexadecimal string, to generate a valid OTP across platforms.
This problem can be easily fixed by converting a hexadecimal string to its binary form before passing it to hash_hmac().
<?php
$time = hex2bin('0000000003523f77'); // time must be in this "hexadecimal and padded" form
$key = hex2bin('bb57d1...'); // 160-bits = 40-digit hexadecimal (4 bits) = 32-digit base32 (5 bits)
hash_hmac('sha1', $time, $key);
?>

Sometimes a hosting provider doesn't provide access to the Hash extension. Here is a clone of the hash_hmac function you can use in the event you need an HMAC generator and Hash is not available. It's only usable with MD5 and SHA1 encryption algorithms, but its output is identical to the official hash_hmac function (so far at least).
<?php
function custom_hmac($algo, $data, $key, $raw_output = false)
{
 $algo = strtolower($algo);
 $pack = 'H'.strlen($algo('test'));
 $size = 64;
 $opad = str_repeat(chr(0x5C), $size);
 $ipad = str_repeat(chr(0x36), $size);
 if (strlen($key) > $size) {
 $key = str_pad(pack($pack, $algo($key)), $size, chr(0x00));
 } else {
 $key = str_pad($key, $size, chr(0x00));
 }
 for ($i = 0; $i < strlen($key) - 1; $i++) {
 $opad[$i] = $opad[$i] ^ $key[$i];
 $ipad[$i] = $ipad[$i] ^ $key[$i];
 }
 $output = $algo($opad.pack($pack, $algo($ipad.$data)));
 return ($raw_output) ? pack($pack, $output) : $output;
}
?>

Example Use:
<?php
custom_hmac('sha1', 'Hello, world!', 'secret', true);
?>

A function implementing the algorithm outlined in RFC 6238 (http://tools.ietf.org/html/rfc6238)
<?php
/**
 * This function implements the algorithm outlined
 * in RFC 6238 for Time-Based One-Time Passwords
 *
 * @link http://tools.ietf.org/html/rfc6238
 * @param string $key the string to use for the HMAC key
 * @param mixed $time a value that reflects a time (unix
 * time in the example)
 * @param int $digits the desired length of the OTP
 * @param string $crypto the desired HMAC crypto algorithm
 * @return string the generated OTP
 */
function oauth_totp($key, $time, $digits=8, $crypto='sha256')
{
 $digits = intval($digits);
 $result = null;
 
 // Convert counter to binary (64-bit) 
 $data = pack('NNC*', $time >> 32, $time & 0xFFFFFFFF);
 
 // Pad to 8 chars (if necessary)
 if (strlen ($data) < 8) {
 $data = str_pad($data, 8, chr(0), STR_PAD_LEFT);
 } 
 
 // Get the hash
 $hash = hash_hmac($crypto, $data, $key);
 
 // Grab the offset
 $offset = 2 * hexdec(substr($hash, strlen($hash) - 1, 1));
 
 // Grab the portion we're interested in
 $binary = hexdec(substr($hash, $offset, 8)) & 0x7fffffff;
 
 // Modulus
 $result = $binary % pow(10, $digits);
 
 // Pad (if necessary)
 $result = str_pad($result, $digits, "0", STR_PAD_LEFT);
 
 return $result;
}
?>

For signing an Amazon AWS query, base64-encode the binary value:
<?php
 $Sig = base64_encode(hash_hmac('sha256', $Request, $AmazonSecretKey, true));
?>

Here is an efficient PBDKF2 implementation:
<?php
/*
 * PBKDF2 key derivation function as defined by RSA's PKCS #5: https://www.ietf.org/rfc/rfc2898.txt
 * $algorithm - The hash algorithm to use. Recommended: SHA256
 * $password - The password.
 * $salt - A salt that is unique to the password.
 * $count - Iteration count. Higher is better, but slower. Recommended: At least 1024.
 * $key_length - The length of the derived key in bytes.
 * $raw_output - If true, the key is returned in raw binary format. Hex encoded otherwise.
 * Returns: A $key_length-byte key derived from the password and salt.
 *
 * Test vectors can be found here: https://www.ietf.org/rfc/rfc6070.txt
 *
 * This implementation of PBKDF2 was originally created by defuse.ca
 * With improvements by variations-of-shadow.com
 */
function pbkdf2($algorithm, $password, $salt, $count, $key_length, $raw_output = false)
{
 $algorithm = strtolower($algorithm);
 if(!in_array($algorithm, hash_algos(), true))
 die('PBKDF2 ERROR: Invalid hash algorithm.');
 if($count <= 0 || $key_length <= 0)
 die('PBKDF2 ERROR: Invalid parameters.');
 $hash_length = strlen(hash($algorithm, "", true));
 $block_count = ceil($key_length / $hash_length);
 $output = "";
 for($i = 1; $i <= $block_count; $i++) {
 // $i encoded as 4 bytes, big endian.
 $last = $salt . pack("N", $i);
 // first iteration
 $last = $xorsum = hash_hmac($algorithm, $last, $password, true);
 // perform the other $count - 1 iterations
 for ($j = 1; $j < $count; $j++) {
 $xorsum ^= ($last = hash_hmac($algorithm, $last, $password, true));
 }
 $output .= $xorsum;
 }
 if($raw_output)
 return substr($output, 0, $key_length);
 else
 return bin2hex(substr($output, 0, $key_length));
}
?>

Simple implementation of hmac sha1
<?php
function hmac_sha1($key, $data)
{
 // Adjust key to exactly 64 bytes
 if (strlen($key) > 64) {
 $key = str_pad(sha1($key, true), 64, chr(0));
 }
 if (strlen($key) < 64) {
 $key = str_pad($key, 64, chr(0));
 }
 // Outter and Inner pad
 $opad = str_repeat(chr(0x5C), 64);
 $ipad = str_repeat(chr(0x36), 64);
 // Xor key with opad & ipad
 for ($i = 0; $i < strlen($key); $i++) {
 $opad[$i] = $opad[$i] ^ $key[$i];
 $ipad[$i] = $ipad[$i] ^ $key[$i];
 }
 return sha1($opad.sha1($ipad.$data, true));
}

The Implementation of the PBKDF2 key derivation function as described in RFC 2898 can be used to not only get the hashed KEY but also a specific IV.
To use, one would use it as follows:-
<?php
 $p = str_hash_pbkdf2($pw, $salt, 10, 32, 'sha1');
 $p = base64_encode($p);
 $iv = str_hash_pbkdf2($pw, $salt, 10, 16, 'sha1', 32);
 $iv = base64_encode($iv);
?>

The function should be:-
<?php
 // PBKDF2 Implementation (described in RFC 2898)
 //
 // @param string p password
 // @param string s salt
 // @param int c iteration count (use 1000 or higher)
 // @param int kl derived key length
 // @param string a hash algorithm
 // @param int st start position of result
 //
 // @return string derived key
 function str_hash_pbkdf2($p, $s, $c, $kl, $a = 'sha256', $st=0)
 {
 $kb = $start+$kl; // Key blocks to compute
 $dk = ''; // Derived key
 // Create key
 for ($block=1; $block<=$kb; $block++)
 {
 // Initial hash for this block
 $ib = $h = hash_hmac($a, $s . pack('N', $block), $p, true);
 // Perform block iterations
 for ($i=1; $i<$c; $i++)
 {
 // XOR each iterate
 $ib ^= ($h = hash_hmac($a, $h, $p, true));
 }
 $dk .= $ib; // Append iterated block
 }
 // Return derived key of correct length
 return substr($dk, $start, $kl);
 }
?>

Function for those, who really need to use crc32 algorithm in PHP>7.1
<?php
 function hash_hmac_crc32(string $key, string $data): string
 {
 $b = 4;
 if (strlen($key) > $b) {
 $key = pack("H*", hash('crc32', $key));
 }
 $key = str_pad($key, $b, chr(0x00));
 $ipad = str_pad('', $b, chr(0x36));
 $opad = str_pad('', $b, chr(0x5c));
 $k_ipad = $key ^ $ipad;
 $k_opad = $key ^ $opad;
 return hash('crc32', $k_opad . hash('crc32', $k_ipad . $data, true));
 }
?>