Update README for PHP sample

Author: boltomli

Samples-Http/PHP/README.md

@@ -7,5 +7,5 @@ XML module for PHP is necessary to build SSML.
 ## Usage
 
 1. Get unified api key for [Free](https://azure.microsoft.com/en-us/try/cognitive-services/?api=speech-services) or [Paid](https://go.microsoft.com/fwlink/?LinkId=872236)
-1. Fill the key to the line in code `$apiKey = "Your api key goes here";`
-1. Run `php TTSService.php` to get the result
+1. Set environment variables `MYREGION` and `MYKEY`
+1. Run `php TTSService.php` to get the result