Sample of processing an SFDC signed request in PHP. To see it in context in in Workbench, see: https://github.com/ryanbrainard/forceworkbench/blob/b9d9ba376c255a8970bc8d10686124747fab9039/workbench/controllers/LoginController.php#L143-L164

gistfile1.php

function processSignedRequest($signedRequest, $secret) {
        $sep = strpos($signedRequest, '.');
        $encodedSig = substr($signedRequest, 0, $sep);
        $encodedEnv = substr($signedRequest, $sep + 1);

        $calcedSig = base64_encode(hash_hmac("sha256", $encodedEnv, $secret, true));

        if ($calcedSig != $encodedSig) {
            throw new Exception("Signed request authentication failed");
        }

        return json_decode(base64_decode($encodedEnv))
}