Add php webhook implementation

[jerryheir]

Pure Php Code

[jerryheir]

On line 5, I made an error of using a variable that doesn't match make with the one in other parts of the code... I updated in another commit by changing it to $accessToken. I also used comment syntax /* and // but some of them will be removed when the code is been put to use

[0sc]

Hey @jerryheir, this looks much more involved that the accompanying js version. What do you think of something like:

<?php
//replace <VERIFY_TOKEN> here with the verify token you specified on messenger dashboard
$hubVerifyToken = '<VERIFY_TOKEN>';

if ($_REQUEST['hub_verify_token'] === $hubVerifyToken){
    echo $_REQUEST['hub_challenge'];
} else {
    echo "Invalid verify token";
}
?>