- hash_hmac: documentation ( source)
<?php
$access_token = 'EAANN7w5edCMBAD57ZBQncnUCzI5ZBmDz0cU2uSTN5bKn4xwBp2nT5CCKnz7gtu3PnNSUXZApceMd60SX1ULsd87XeWh7YDgFj7m77FKnMQkPz28OTmHzwdTXaURg2OIJ4c1ZCUfTXsDc9E52GZANqCvKXKzqRZAKkCPwfDE5UXJwZDZD';
$app_secret = '4ee65649196bbe545409e15eb1e6b264';
$appsecret_proof= hash_hmac('sha256', $access_token, $app_secret);
echo $appsecret_proof;