Migrate skip checks to --EXTENSIONS--, p3

For rationale, see #6787

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

Implement Explicit octal notation for integers RFC

RFC: https://wiki.php.net/rfc/explicit_octal_notation

Add an extensive test suits for other variants of integer literals

Implement Explicit octal notation for integers RFC

RFC: https://wiki.php.net/rfc/explicit_octal_notation

Add an extensive test suits for other variants of integer literals

Closes GH-6360

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