通过URL上传图片

Code:

if(isset($_POST['update_avatar'])) {
    $url = $_POST['avatar'];
    $info = getimagesize($url);

    if(isset($info['name'])) {
        echo "Exists";
    } else {
        echo "Error";
    }
}

How can I avoid getting PHP errors when the user types an invalid URL, random piece of text or invalid image URL etc?

If there's an error, getimagesize returns false, so test for that. And to suppress error messages from a function, put @ before it:

$info = @getimagesize($url);
if (!$info) {
    echo "Error";
} else {
    // Process the image
}

Use exception handling. Place your critical code into a try..catch block. You can find more information here.