Class to get a list of all CP releases and to get the migration URL from a release.

class-classicpress-releases.php

<?php
class ClassicPressReleases {
	/**
	 * Get migration URL fetching created_at from api.github.com.
	 *
	 * @param string $version  Version to retrive migration URL.
	 *
	 * @return string|bool     URL for release. Empty string if no release found. False on API failure.
	 */
	public static function getMigrationFromCPVersion($version) {
		$response = wp_remote_get('https://api.github.com/repos/ClassicPress/ClassicPress-release/releases/tags/'.$version, ['timeout'=>3]);
		if (is_wp_error($response) || empty($response)) {
			return false;
		}

		$data = json_decode(wp_remote_retrieve_body($response), true);
		if(isset($data['message']) && $data['message'] === 'Not Found') {
			return '';
		}

		$created_at = new \DateTime($data['created_at']);
		$day        = $created_at->format('Ymd');
		$exploded   = explode('.', $version);
		$major      = $exploded[0];
		$url        = 'https://github.com/ClassyBot/ClassicPress-v'.$major.'-nightly/releases/download/'.$version.'%2Bmigration.'.$day.'/ClassicPress-nightly-'.$version.'-migration.'.$day.'.zip';
		return $url;
	}

	/**
	 * Get release URL.
	 *
	 * @param string $version  Version to retrive migration URL.
	 *
	 * @return string          URL for release.
	 */
	public static function getReleaseFromCPVersion($version) {
		return 'https://github.com/ClassicPress/ClassicPress-release/archive/refs/tags/'.$version.'.zip';
	}

	/**
	 * Get a list of ClassicPress released versions from api-v1.classicpress.net.
	 *
	 * @return array|bool Array of CP versions or false on API failure.
	 */
	public static function getCPVersions() {
		$response = wp_remote_get('https://api-v1.classicpress.net/v1/upgrade/index.php', ['timeout'=>3]);
		if (is_wp_error($response) || empty($response)) {
			return false;
		}
		$versions = json_decode(wp_remote_retrieve_body($response));

		// Get only stable releases
		foreach ($versions as $key => $version) {
			if(!str_contains($version, 'nightly') && !str_contains($version, 'rc') && !str_contains($version, 'alpha') && !str_contains($version, 'beta')) {
				continue;
			}
			unset($versions[$key]);
		}

		// Strip .json from version
		$versions = array_map(
			function($v) {
				return substr($v, 0, -5);
			},
			$versions
		);

		// Sort using SemVer
		usort($versions, 'version_compare');

		return array_values($versions);
	}

	/**
	 * Get previous release version.
	 *
	 * @param string $version   Version to get previous release.
	 * @param array  $versions  Array of ClassicPress versions as
	 *                          returned by getCPVersions().
	 *                          Used for caching.
	 *
	 * @return string|bool      Previous version. False if not found.
	 */
	public static function previous($version, $versions = []) {
		if (empty($versions)) {
			$versions = self::getCPVersions();
		} else {
			usort($versions, 'version_compare');
		}
		if(!in_array($version, $versions)) {
			return false;
		}
		$pos = array_search($version, $versions, true);
		if(!isset($versions[$pos - 1])) {
			return false;
		}
		return $versions[$pos - 1];;
	}

	/**
	 * Get latest release version URL.
	 *
	 * @param string $major     Major release. Default 2.
	 * @param array  $versions  Array of ClassicPress versions as
	 *                          returned by getCPVersions().
	 *                          Used for caching.
	 *
	 * @return string|bool      Latest elease URL.
	 */
	public static function latest($major = '2', $versions = []) {
		if (empty($versions)) {
			$versions = self::getCPVersions();
		} else {
			usort($versions, 'version_compare');
		}
		foreach ($versions as $index => $version) {
			$exploded = explode('.', $version);
			if ($exploded[0] === $major) {
				continue;
			}
			unset($versions[$index]);
		}
		return array_pop($versions);
	}

	/**
	 * Compare two versions.
	 *
	 * @param string $v1  Version to compare.
	 * @param string $v1  Version to compare.
	 *
	 * @return string     Major, minor, patch or equal.
	 */
	public static function compare($v1, $v2) {
		if ($v1 === $v2) {
			return 'equal';
		}
		$v1_split = explode('.', $v1);
		$v2_split = explode('.', $v2);
		if ($v1_split[0] !== $v2_split[0]) {
			return 'major';
		}
		if ($v1_split[1] !== $v2_split[1]) {
			return 'minor';
		}
		return 'patch';
	}
}