Saving graphic to local folder from cron job - php

Question

I have a cron job that runs once a week and checks a remote site for any updates, which may include new data, and may include new graphics.
I check my local folder to see if the graphic already exists, and if it doesnt then I want to save a local copy of that graphic.
My code seems to work when I run it from the browser, but when I check the folder every week, there are a lot of empty graphic files.
They have the correct filenames, but are all zero bytes.
My code inside the php file that the cron job runs is this:
if (!file_exists($graphic)) {
$imageString = file_get_contents($graphicurl);
file_put_contents($graphic, $imageString);
}
$graphic will be something like "filename.jpg"
$graphicurl will be something like "https://remotegraphicfile.jpg"
And I see many files such as "filename.jpg" that exist in my local folder, but with a zero byte filesize.
Is there any reason why this wouldnt work when called by a cron job?

Answer 1

I have now managed to get this working, by changing my original code from:
if (!file_exists($graphic)) {
$imageString = file_get_contents($graphicurl);
file_put_contents($graphic, $imageString);
}
To this:
if (!file_exists($graphic)) {
$file = fopen ($graphic, 'w');
$c = curl_init($graphicurl);
curl_setopt($c, CURLOPT_FILE, $file);
curl_exec($c);
curl_close($c);
fclose($file);
}
This now works, and graphics save to my folder when running the cron job.