Context

Trying an app on the next PHP before it's released catches breakage early, but PHP's pre-releases (alpha, beta and RC) aren't where its releases are. The tarballs live in each release manager's folder on downloads.php.net, listed with their SHA-256 in php.net's pre-release-builds.php?format=json; static-php.dev has no builds of them. On Windows, windows.php.net publishes them as QA builds, in their own downloads/qa/releases.json with a SHA-256 for each zip. Extensions lag behind a new minor: many PECL extensions don't compile against it yet, and Xdebug supports it only once it's close to release. A preview also mustn't land on a site by accident, through Make default or a composer.json constraint such as ^8.2.

Decision

Which versions. runtimes::php::PREVIEW lists the minors offered as previews (8.6 today). SUPPORTED stays released versions only: only those can be the default, be suggested, or be picked from composer.json while one fits. ALL_MINORS is PREVIEW then SUPPORTED, and installing, listing and updating use it.
Never the default, pinnable per site. Make default and set_php_default refuse a preview, and the tray leaves previews out of its default picker. A site gets a preview only when it's pinned to it on its Site settings (the picker says "8.6 (preview)"), or when its require.php fits no installed released version but fits the preview.
macOS: Short Leash's own build. ShortLeash/runtimes builds the newest php.net pre-release of each minor in its workflow's PHP_PREVIEW_MINORS. Its plan reads php.net's pre-release list, takes only a .tar.xz on https://downloads.php.net/ named for that version, and the build checks the tarball against the SHA-256 the list publishes before static-php-cli sees it. The release is a GitHub pre-release tagged like php-8.6.0RC3, attested and published in the signed manifest like Short Leash's other builds (0017, 0019), filed under php/8.6.0RC3/.
Windows: windows.php.net's QA builds. When a PREVIEW minor isn't in the stable releases.json, Short Leash fetches downloads/qa/releases.json once, takes only the PREVIEW minors from it (not the RCs of released minors it also lists), and checks the zip against its SHA-256. The compiler is picked by number, so 8.6's vs18 build is taken.
Naming. Versions keep php.net's names (8.6.0RC3, 8.6.0beta1). runtimes::updates::version_key sorts alpha before beta before RC before the release, so a newer RC replaces the installed one as an update, and so does the release.
No Xdebug for a preview. Its Xdebug switch is disabled ("Not available for previews yet"), set_xdebug(.., true) refuses one before anything is downloaded, and a site's extension check never tells you to turn Xdebug on for it.
Extensions it lacks. Extensions that don't build against the preview yet go in ShortLeash/runtimes' scripts/php-extensions-preview-skip.txt, each with a reason; runtimes::php::PREVIEW_MISSING_EXTENSIONS mirrors it, so Short Leash never offers them for a preview on macOS. For 8.6.0RC3 that's 14: apcu, ev, event, excimer, grpc, igbinary, imagick, memcached, msgpack, pcov, pdo_sqlsrv, redis, sqlsrv and swoole. On Windows a preview has what windows.php.net's zip holds, and Short Leash adds no extension DLLs for it.
When the release ships. A Short Leash release moves the minor from PREVIEW to SUPPORTED and empties the skip list (the builds skill has the steps); installed previews update to the release.

Consequences

People can run their app on the next PHP weeks before release, from the Runtimes screen, without building PHP or adding a package source.
A preview build has fewer extensions than a released one until upstream catches up, and the docs list which. Apps that need one of them, or Xdebug, wait for the release.
Release candidates replace each other as updates, so a site pinned to 8.6 moves from RC to RC without asking; that's what a preview is for, but an RC can break what the last one ran.
Windows reaches one more source, windows.php.net's QA listing, only while a preview minor is missing from the stable one. The Privacy policy says so. CLAUDE.md invariant 7 names both checks.
Moving a minor out of PREVIEW is a code change and a Short Leash release, not a manifest update: a release can't become the default before Short Leash knows it's released.