Simple validation function to confirm that an image is actually an image, not a malicious .gif or .php file.

Image Upload Validation

function validate($upload){ // Must be a $_FILES array
  if($upload['size'] == 0) return "Image not uploaded correctly.";
  if($upload['size'] > 2097152){ // Measured in bytes, this is equal to 2MB
    $filesize = $upload['size']/1048576; // Converts from bytes to Megabytes
    return "The image you uploaded has a filesize that is too large. Please reduce your image to < 2MB. It is currently ".$filesize."MB.";
  }
  if(($upload['type'] != "image/gif" || "image/jpeg" || "image/png") || ($this->imageinfo['mime'] != "image/gif" || "image/jpeg" || "image/png"))
    return "Uploads of that file type are not allowed. You need a jpg, png, or gif image."; 
  $blacklist = array(".php", ".phtml", ".php3", ".php4", ".ph3", ".ph4");
  foreach ($blacklist as $item) {
    if(preg_match("/$item\$/i", $upload['name']))
      return "Uploads with that file extension are not allowed. You need an image ending in .jpg, .png, or .gif";
  }
  return true; // Validates
}