Also it seems that return value is not correctly documented for older PHP versions either. Documentation claims it returns string or false but if you pass an invalid algorithm it actually returns NULL on PHP 7.3 and 7.4.

https://3v4l.org/vugc3

> Documentation claims it returns string or false but if you pass
> an invalid algorithm it actually returns NULL on PHP 7.3 and 7.4.

From the docs[1]:

| If the parameters given to a function are not what it expects,
| such as passing an array where a string is expected, the return
| value of the function is undefined. In this case it will likely
| return null but this is just a convention, and cannot be relied
| upon.

[1] <https://www.php.net/manual/en/functions.internal.php>