PHP create JSON file from Twitter API
This is a partial review.

$file_loc and $json_folder

Use $json_folder in the construction of $file_loc. If you decide to use a different folder, you need change two variables.

$file_loc = "{$json_folder}/twitter.json";


Naming of function

You named your function get_twitter_json()... which... does essentially nothing. It does not return json like the function name suggests. It doesn't even return the twitter result. Instead this seems a partial implementation of a processing function. If it is supposed to return tweets, then name at least something like get_tweets().

Whitespace

Use consistent whitespace. Trying to align all assignments to the same indentation will create problems sooner than later. Changing one assignment will mess up the rest. Multi-line assignments will look even weirder. I would recommend 1 space before the assignment operator (=) and one space after it.

$results and $tweets

The if-else-statement should have some code behind it. The if-statement loads the decoded json-file in $results, while the else-statement loads it in $tweets. Use one of them.

While you are at it, while php allows you to use $tweets undefined like that, it doesn't make for clear code.

foreach with a counter

You are using a foreach loop with a custom counter. That... defeats the purpose of a foreach loop. Either use a for loop and use the exposed counter from that, or use the append syntax to don't need a counter.

foreach($results as $tweet) : 
    $new_tweet = array();
    $new_tweet['created']      = $tweet['created_at'];
    $new_tweet['text']         = $tweet['text'];
    $new_tweet['username']     = $tweet['user']['name'];
    $new_tweet['screenname']   = $tweet['user']['screen_name'];
    $new_tweet['url']          = $tweet['user']['url'];
    $tweets[] = $new_tweet;
endforeach;