为什么IMAGETYPE_JPEG不接受所有JPEG？

Question

I am confused about why this code says JPEG files and PNG files are invalid. I want this code to only accept JPEG and PNG, and deny all other file types, but right now, it's denying everything.

<?

echo '
    <form method="post" action="upload.php">
        <input type="text" placeholder="url" name="url" /> <input type="submit" value="check" name="submit" />
    </form>
';

if (isset($_POST["submit"])) {

$url = $_POST["url"];

echo '<strong>URL:</strong> ' . $url;
echo '<br /><br />';

if(!filter_var($url, FILTER_VALIDATE_URL)) { //not valid f-in url
  echo('Invalid url given'); 

} else {
    if (exif_imagetype($url) != IMAGETYPE_JPEG || exif_imagetype($url) != IMAGETYPE_PNG) {
        echo 'Invalid Image<br />';
    } else {
        echo "Works.";
    }
}

}
?>

Answer 1

Your statement logic is backwards. This is essentially what you're writing:

$image_type = exif_imagetype($url);
if ($image_type != IMAGETYPE_JPEG || $image_type != IMAGETYPE_PNG) {
    echo 'Invalid Image<br />';
} else {
    echo "Works.";
}

Which translates to "if the image type isn't a JPEG or isn't a PNG, it's invalid". Instead, you want:

if( $image_type == IMAGETYPE_JPEG || $image_type == IMAGETYPE_PNG) {
    echo "Works.";
} else {
    echo "Invalid Image<br />";
}

Which is "if the image type is JPEG or PNG, it's valid", or:

if ($image_type != IMAGETYPE_JPEG && $image_type != IMAGETYPE_PNG) {
    echo 'Invalid Image<br />';
} else {
    echo "Works.";
}

"if the image isn't JPEG and it isn't PNG, it's invalid".