base64 decode mixed results - php

Question

I have the following code in PHP, and it works for the most, fine. I am sending a image from a mobile device to this script, which decodes it into a img file and creates a file out of it on the server. I am 99.9% sure every time its a base64 encoded.
<?php
header('Access-Control-Allow-Origin: *');
header('Content-Type: image/jpeg');
$data = ($_POST['imageData']);
define('UPLOAD_DIR', 'images/');
$img = str_replace('data:image/jpeg;base64,', '', $data);
$data = base64_decode($img);
$file = UPLOAD_DIR . uniqid() . '.jpg';
file_put_contents($file, $data);
echo ('{"imgUrl" : "' . $file . '"}');
?>
This then returns the image URL back to be added to a database.
The problem is, most of the time it does decode into a .jpg file, and other times into a txt file. I cannot see why it does it, as its a little random. But I have noticed that sometimes it will come as a $_POST, and other times, $_POST is Null. So I looked at using:
$data = json_decode(file_get_contents('php://input'));
But again, it seems inconsistant. But I put a logic statement such as:
$data = ($_POST['imageData']);
if($data == NULL) {
$data = json_decode(file_get_contents('php://input'));
}
Is there any reason I should be aware of why the code works, and sometimes does not work ?

Answer 1

I know this question is old, but is one of the first appearance by looking at this topic. So everyone looking at this question can find a link to a proper answer right away.
You should check this PHP - get base64 img string decode and save as jpg (resulting empty image )
Also check the conditions you're using, because
if ($data === NULL)
it may be different for
if ($data == NULL)
Also, you're saving the base64 string incorrectly to an image file.
Check that link and let me know how if it helped.