Error decoding JSON in PHP: "unexpected character" - php

Question

I have a strange JSON decoding problem in PHP. A certain JSON file (and only this) produces reliably a decoding error. The PHP part is nothing special:
$data = array();
if (is_file(DIR.'config.json')) {
$data = json_decode(DIR.'config.json');
}
// diagnostics:
var_dump($data); // --> NULL (would be array() if decoding didn't happen)
var_dump(json_last_error()); // --> int(4) === JSON_ERROR_SYNTAX
var_dump(json_last_error_msg()); // --> string(20) "unexpected character"
echo file_get_contents(DIR.'config.json'); // --> '["a"]'
What I've tried:
Simplify JSON. The content of the file now is ["a"].
validate JSON, just to be sure, in four validators, including jsonlint.com and pasting it in browser console
No BOM, plain ASCII file
has read rights. The above echo works just fine.
completely independent of JSON file's content.
path is correct. Proven by above echo statement. JSON file is in the docroot.
Unfortunately PHP's JSON lib doesn't state errors more precisely, so I have no idea, what really causes the error.
Has anyone an idea, how I could proceed?

Answer 1

You're not decoding the file contents, but rather the name of the file itself, which is why you're getting null. Your code should be:
$data = array();
if (is_file(DIR.'config.json')) {
$data = json_decode(file_get_contents(DIR.'config.json'));
}

Answer 2

json_decode(DIR.'config.json');
You are trying to decode the string "<whatever-DIR-is>config.json". That's not a valid JSON string. You probably want:
json_decode(file_get_contents(DIR.'config.json'));
You also probably want more coffee.