You need a copy of the PHP wrapper library file TMRestClient.php, available from here.
You should place the file in your class path or in the folder where you are executing the code.
You must then 'include' the library somewhere in your PHP code, before you can use it:
include_once('TMRestClient.php');
If the library file is not in your include path, you can instead specify the absolute/relative path in the include_once argument:
include_once('/my/path/to/my/code/TMRestClient.php');