crypt() function behavior – is that a php bug?

working with crypt() function, I can observe that :

$var1 = crypt('rasmuslerdorf', 'cd');
$var2 = crypt('rasmuslerdorfWhatever', 'cd');

generates exactly the same output ! Here : cdqYxeZGYiRPw

You can try by yourself online : https://fr.functions-online.com/crypt.html

From the doc I can read about the maximum length of the resulting string, but nothing about the input string !

Returns the hashed string or a string that is shorter than 13 characters and is guaranteed to differ from the salt on failure.

Is that a normal behavior ? I don’t think so.

I’m using PHP Version 8.1.2