$file_encoded = '[very LONG BASE64 STRING]'; $file_decoded = base64_decode($file_encoded); $info = getimagesizefromstring($file_decoded); print_r($size);
If the decoded data is a valid image, the getimagesizefromstring() function will return the following info if said string is a proper image:
Array (  => 240  => 464  => 3  => width="240" height="464" [bits] => 8 [mime] => image/png )
Which makes it perfect for image validation. It returns FALSE if it's not able to parse the input as an image.
You'll need to have the GD image library installed to use this function.