Migrate skip checks to --EXTENSIONS--, p3

For rationale, see #6787

Extensions migrated in part 3:
* ftp
* gmp
* iconv
* opcache
* shmop

Improve undefined variable error messages

Closes GH-5312

Fix $x = (bool)$x; for undefined with opcache

And `$x = !$x`

Noticed while working on GH-4912

The included test would not emit undefined variable errors in php 8.0
with

Fix $x = (bool)$x; for undefined with opcache

And `$x = !$x`

Noticed while working on GH-4912

The included test would not emit undefined variable errors in php 8.0
with opcache enabled. The command used:

```
php -d zend_extension=opcache.so --no-php-ini -d error_reporting=E_ALL \
-d opcache.file_cache= -d opcache.enable_cli=1 test.php
```

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