Issue #2917655 by xjm, Spokje, catch, Gábor Hojtsy, Pasqualle, rivimey,...

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Prerequisites:

- PHP 7.3.0 (or greater) (https://php.net).

- PHP 7.4.0 (or greater) (https://php.net).

In the instructions below, replace the version x.y.z with the specific version

you wish to download. Example: 8.6.0.zip. You can find the latest stable version

- A web server with PHP support, for example:

  - Apache 2.4.7 (or greater) (http://httpd.apache.org/).

  - Nginx 1.1 (or greater) (http://nginx.com/).

- PHP 7.3.0 (or greater) (http://php.net/). For better security support it is

  recommended to update to at least 7.3.13.

- PHP 7.4.0 (or greater) (http://php.net/). For better security support it is

  recommended to update to at least 8.1.0.

- One of the following databases:

  - MySQL 5.7.8 (or greater) (http://www.mysql.com/).

  - MariaDB 10.3.7 (or greater) (https://mariadb.org/). MariaDB is a fully

// hardcoded minimum PHP version below (both in the version_compare() call and

// in the printed message to the user) whenever \Drupal::MINIMUM_PHP is

// updated.

if (version_compare(PHP_VERSION, '7.3.0') < 0) {

  print 'Your PHP installation is too old. Drupal requires at least PHP 7.3.0. See <a href="http://php.net/supported-versions.php">PHP\'s version support documentation</a> and the <a href="https://www.drupal.org/docs/system-requirements/php-requirements">Drupal PHP requirements</a> page for more information.';

if (version_compare(PHP_VERSION, '7.4.0') < 0) {

  print 'Your PHP installation is too old. Drupal requires at least PHP 7.4.0. See <a href="http://php.net/supported-versions.php">PHP\'s version support documentation</a> and the <a href="https://www.drupal.org/docs/system-requirements/php-requirements">Drupal PHP requirements</a> page for more information.';

  exit;

}

   *   that Drupal no longer supports that PHP version.

   * - An error is shown in the status report that the PHP version is too old.

   */

  const MINIMUM_SUPPORTED_PHP = '7.3.0';

  const MINIMUM_SUPPORTED_PHP = '7.4.0';

  /**

   * Minimum allowed version of PHP for Drupal to be bootstrapped.