PHP整数参数，例如curl_setopt和filter_var

Question

I've been browsing the documentation relentlessly, but I just can't seem to figure out what it's called when a function uses an argument which has an int datatype. For example, in filter_var:

filter_var('[e-mail]', FILTER_VALIDATE_EMAIL);

... or in curl

curl_setopt($curl, CURLOPT_URL, 'http://example.com');

What is FILTER_VALIDATE_EMAIL and CURLOPT_URL called?

Answer 1

They are called Constants.

It just a variable, but because it is constant it should be written as Capital letters just to distinguish from other types of variables.That's the naming convention.

define('YOUR_CONSTANT', '894616466');
echo YOUR_CONSTANT;//894616466