As per the PHP manual:
> If the `component` parameter is omitted, an associative array is returned.
> If the `component` parameter is specified, `parse_url()` returns a string (or an int, in the case of `PHP_URL_PORT`) instead of an array. If the requested component doesn't exist within the given URL, `null` will be returned.
Reference: [https://www.php.net/manual/en/function.parse-url.php#refsect1-function.parse-url-returnvalues PHP Manual: parse_url(): Return Values]
This commit adds three unit tests for `download_url()`:
* The first test is "girl-scouting" to make sure that the code up to the point where the error is expected is tested.
* The second test exposed a PHP 8.1 `basename(): Passing null to parameter #1 ($path) of type string is deprecated` error due to the call to `parse_url()` returning `null` when the component requested does not exist in the passed URL.
* The output of the call to `parse_url()` stored in the `$url_path` variable is used in more places in the function logic. The third test exposes a second PHP 8.1 deprecation notice, this time for `substr(): Passing null to parameter #1 ($string) of type string is deprecated`.
This commit also removes duplicate `parse_url()` calls. Neither `$url` nor `$url_filename` are changed between when they are first received/defined and when they are re-used, so there is no need to repeat the function calls.
Follow-up to [51606], [51622].
Props jrf, hellofromTonya, SergeyBiryukov.
See #53635.
Built from https://develop.svn.wordpress.org/trunk@51626
git-svn-id: http://core.svn.wordpress.org/trunk@51232 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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