here is the way we compute digests and compare them with the digests compted by Epoch

epochDigest.php

<?php
	public function getDigest($flexpost, $source_encoding=null) {
		if ( array_key_exists('epoch_digest', $flexpost) )
			unset($flexpost['epoch_digest']);
		ksort($flexpost);

		$sorted_string = urldecode(strtr(http_build_query($flexpost), ['&' => '', '=' => '']));
		if ( $source_encoding )
			$sorted_string = mb_convert_encoding($sorted_string, self::UTF8_ENCODING, $source_encoding);
		return hash_hmac('md5', $sorted_string, $this->hmac_key);
		}

	public function checkDigest($flexpost) {
		$digest = $flexpost['epoch_digest'];
		if ( $digest == $this->getDigest($flexpost) )
			return self::UTF8_ENCODING;
		if ( $digest == $this->getDigest($flexpost, self::WINDOWS_ENCODING) )
			return self::WINDOWS_ENCODING;
		throw new Exception("Bad security digest");
		}