This PR was merged into the 2.3 branch.

Discussion
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[Console] use PHP_OS instead of php_uname('s')

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #15058
| License       | MIT
| Doc PR        |

The php_uname() function may be disabled for security reasons.

Commits
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40e0dc8 use PHP_OS instead of php_uname('s')

…hnkary)

This PR was squashed before being merged into the 2.3 branch (closes #16095).

Discussion
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[Console] Add additional ways to detect OS400 platform

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #16053
| License       | MIT
| Doc PR        | None

This PR adds support for detecting the OS400 platform when the PHP function `php_uname()` is disabled. OS400 platform detection was added in #15058 to fix character encoding issues present on OS400. See that PR for more info.

This PR fixes regression introduced in #16053, which did not work on the IBM OS400 server I have access to. The constant `PHP_OS` being checked outputs "AIX" on my IBM OS400 server. I can't say for sure if it works on other IBM platforms... but I preserved this check just in case.

User @eloigranado [commented here](#15058 (comment)) asking if we could switch to using `PHP_OS` constant instead of `php_uname()` because he claims some admins might "[hide] the exact kernel build from any attacker who discovers a remote PHP code execution vulnerability". I personally don't think we should accommodate this use case, but I was able to find alternate approaches.

### Why use case insensitive string matching stristr() instead of in_array()?

Here are the various outputs on my OS400 server:

    echo PHP_OS; // "AIX"
    echo getenv('OSTYPE'); // "os400"
    echo php_uname('s'); // "OS400"

So we have various case issues here, and possible blank values on platforms where OSTYPE var doesn't exist or php_uname() is disabled. Concatenating these optional values together delimited by ; then case-insensitive searching the string for "OS400" seemed like a fair compromise. I would've probably done `in_array()` if case wasn't an issue.

Commits
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96a4071 [Console] Add additional ways to detect OS400 platform