THRIFT-6005: Raise PHPStan level from 5 to 6 on PHP library

[sveneld]

Step lib/php/phpstan.neon level from 5 to 6 and address every issue PHPStan surfaces along the way. Final phase of the umbrella ticket THRIFT-5960 PHP modernization (after THRIFT-6000 / THRIFT-6001 / THRIFT-6004).

Library typing fixes

• TBinarySerializer::serialize(object $object): string
• TBinarySerializer::deserialize(string $string_object, class-string $class_name, int $buffer_size = 8192): object
• TServer::__construct(object $processor, …)
• TSocket / TSSLSocket constructor: documented $debugHandler as callable|string|null via PHPDoc.
• TSocketPool::__construct(array $hosts, int|array $ports, bool $persist, …) with list<string> / int|list<int> shape PHPDoc.
• TJSONProtocol static array properties get a native array declaration plus @var element-type docblocks ($JSON_CHAR_TABLE, $ESCAPE_CHARS, $ESCAPE_CHAR_VALS).
• TBase / TException internal helpers (readMap, readList, readStruct, writeMap, writeList, writeStruct, __construct, bridge ctor) gain @param array<…> shape documentation so PHPStan can reason about $spec / $vals / $var element types.

Bug fixes

• TBase::__wakeup() previously round-tripped through $this->__construct(get_object_vars($this)). That call short-circuited inside the body (second parameter was null) but tripped strict-mode type checks under tighter array-shape PHPDoc. PHP restores object state automatically on unserialize, so the body is now empty with a comment.
• ThriftClassLoader::findFileInApcu() now uses a clear two-step fetch-or-recompute pattern and narrows the return via is_string(), eliminating the unreachable else-arm flagged by PHPStan.

Baseline

Regex pattern updated for the four unreachable dead-code arms in TBase / TException writeMap that mirror Thrift IDL map<Struct, V> semantics PHP arrays cannot represent — phpstan now reports the type as array<int|string, mixed> instead of bare array.

Validation (Docker thrift-php-dev:local)

• phpcs — 0 errors
• phpstan (level 6) — 0 errors
• phpunit Unit Suite — 637 tests, 0 failures
• phpunit Integration Suite — 108 tests, 0 failures

Closes the typing trajectory started by THRIFT-5960.

• Apache Jira ticket — THRIFT-6005
• PR title follows the pattern "THRIFT-NNNN: …"
• Squashed to a single commit
• No behaviour changes: __wakeup no-op was already a no-op due to body short-circuit; findFileInApcu returns the same string/null contract; library API surface untouched.

Generated-by: Claude Opus 4.7