docs(php-modernization): strict-rule bans (short-ternary, @-suppression, mixed cast)

skills/php-modernization/references/phpstan-compliance.md

@@ -172,6 +172,38 @@ public function getItems(): array
 public function getConfig(): array
 ```
 
+### Strict-Rule Bans: short-ternary, `@`-suppression, `(string) $mixed`
+
+Level 9/10 plus a strict ruleset (`phpstan-strict-rules`, or the `ergebnis`
+custom rules) reject three idioms that pass at lower levels. New code trips them
+repeatedly; the compliant rewrites:
+
+```php
+// ternary.shortNotAllowed — the short ternary is banned:
+$rows = glob($p) ?: [];                       // ✗
+$rows = glob($p);                             // ✓
+if ($rows === false) { $rows = []; }
+
+// ergebnis.noErrorSuppression — the "@" operator is banned. Wrap the
+// warning-emitter and handle the failure explicitly (also catches the case
+// where an error handler turns the warning into an exception):
+$h = @proc_open($cmd, $spec, $pipes);         // ✗
+$h = false;                                   // ✓ init first, so the check is safe if proc_open throws
+try { $h = proc_open($cmd, $spec, $pipes); }
+catch (\Throwable $e) { /* degrade */ }
+if (!\is_resource($h)) { /* degrade */ }
+// For unlink/file ops, guard instead of suppress: if (is_file($f)) { unlink($f); }
+
+// cast.string — casting a mixed to string is unsafe (it could be an array):
+$s = (string) $config->get($key);             // ✗
+$raw = $config->get($key);                    // ✓ string-only intent (config keys);
+$s = \is_string($raw) ? $raw : '';            //   use is_scalar($raw) ? (string) $raw : '' to keep int/bool
+```
+
+Run the static analyzer **after** adding the test files, not before — tests are
+analyzed too, so a `(string) $mixed` cast or an offset-on-possibly-empty in a
+fresh test file fails CI even though the pre-test run was green.
+
 ## Type Aliases and Custom Types
 
 ### Defining Type Aliases