This has been asked before, but most, if not all answers, were either solutions that didn't work in all situations or were unnecessary (like using getimagesize(), which downloads the entire image).
How would I check if a given URL leads to an image without having to hardcode image extensions (like .png', .jpg, etc.)?
You can read only file header by CURL and then detect if the header is image header or not.
curl = curl_init();
curl_setopt($curl, CURLOPT_URL, $file_url);
curl_setopt($curl, CURLOPT_NOBODY, true);
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
$header = curl_exec($curl);
curl_close($curl);
if (strstr($header, 'image/png')) {
//file is image
}