Use core/composer.json to get minimum PHP in install.php

Has to go on 10.0.x first.

Here's the contents on \Drupal\Tests\Composer\ComposerTest::testEnsurePhpConfiguredVersion

  /**
   * Ensure that the configured php version matches the minimum php version.
   *
   * Also ensure that the minimum php version in the root-level composer.json
   * file exactly matches \Drupal::MINIMUM_PHP.
   */
  public function testEnsurePhpConfiguredVersion() {
    $composer_json = json_decode(file_get_contents($this->root . '/composer.json'), TRUE);
    $composer_core_json = json_decode(file_get_contents($this->root . '/core/composer.json'), TRUE);
    $this->assertEquals(\Drupal::MINIMUM_PHP, $composer_json['config']['platform']['php'], 'The \Drupal::MINIMUM_PHP constant should always be exactly the same as the config.platform.php in the root composer.json.');
    $this->assertEquals($composer_core_json['require']['php'], '>=' . $composer_json['config']['platform']['php'], 'The config.platform.php configured version in the root composer.json file should always be exactly the same as the minimum php version configured in core/composer.json.');
  }

As we can see the assumption made about the contents of the require.php key are tested...

+++ b/core/install.php
@@ -23,14 +23,13 @@
+// result from loading the autoloader or core/lib/Drupal.php. The format in
+// composer.json is assumed to be ">=8.1.0".
+$minimum_version = substr(json_decode(file_get_contents(__DIR__ . '/composer.json'), TRUE)['require']['php'], 2);

So the substr() here is safe to do.

I think the notes to remember to update both are covered by the line added:

Note: This is duplicated as a composer constraint in core/composer.json.

And the existing test that's shown in #4 makes it impossible for them to get out of sync.

I like the idea in #7 of deprecating \Drupal::MINIMUM_PHP - I think we should do this in a follow-up because I'd want to focus on any performance concerns. The constant is used when any instance of ModuleExtensionList, ThemeExtensionList etc... are created. I don't think any of these are on the performance critical path but reading the json will definitely not be as cheap as the constant and potentially during a site install these thing might be created a lot.