gistfile1.php

<?php

  // Get the "API_SECRET" variable from our shell session
  $api_secret = getenv('API_SECRET');

  // Same as above, but using a Superglobal
  $api_secret = $_ENV['API_SECRET'];

  // You can use phpinfo to show all the environmental variables you can use.
  phpinfo(INFO_ENVIRONMENT);

  /*
    To set environmental variables you can do:
   
    Linux/BSD: 
    API_SECRET=SOMESUPERSECRETKEYHERE
    Note: you may need to export the key by prepending `export` to the above

    Windows:
    set API_SECRET=SOMESUPRTSECRETKEYHERE
  */ 

?>

Once you do this, you don't have to worry about pushing secret keys to Git, you just have to create the environmental variables in the same shell session as when you run the script.