Global PHP App Configuration Constants & "Includes" -

i creating/re-building sizable php app/website uses few different api's , interfaces database. , so, have store api keys, app keys , other such configuration data centrally accessible of different files/pages.

currently, i'm declaring variables in global context , including them using require_once():

<?php //config.php $api_key['somename']    =    "this_is_my_api_key"; $api_key['othername']   =    "another_api_key";  $app_id['somename']     =    "app_id_here"; $app_id['othername']    =    "other_app_id";  //so on , forth, of api data need, api urls //also, i'm declaring things this...  $__sitewide_killswitch  =    false; $__debug_mode           =    false; $__display_errors       =    false; ?> 

then, in other files, reference these variables this:

require_once('path/to/config.php'); 

only downside if have reference them in functions, since not superglobals $_globals or $_session variables, instance, have this:

function database_connect( ... ) {     global $api_key, $app_id, $api_url; //etc...     ...     ... } 

i want avoiding passing in api key's in through function if can. hoping best practices on declaring site-wide global constants , maybe avoid calling global keyword @ beginning of each function.

should using const in declarations? benefit in using define() function?

am doing right thing require_once() or there better way this?

what think? how should doing this? thanks!