feature #576 Enforce PHP version at runtime to be same or higher as at "composer update" time (nicolas-grekas)

This PR was merged into the 1.4-dev branch.

Discussion
----------

Enforce PHP version at runtime to be same or higher as at "composer update" time

When e.g. PHP 7.3 is used to compute dependencies, here is the message that is now generated when running `bin/console` or `symfony serve` *on PHP 7.1*:

```
Fatal Error: composer.lock was created for PHP version 7.3 or higher but the current PHP version is 7.1.33.
```

We have too many reports on symfony/symfony about ppl that mess up with these.

It works by adding a simple *static* check in `vendor/autoload.php`.

Note that `composer install` already yells, so it's already covered.

Commits
-------

e39f329 Enforce PHP version at runtime to be same or higher as at "composer update" time

Author: fabpot

src/Flex.php

@@ -268,6 +268,13 @@ public function configureInstaller()
         return $backtrace;
     }
 
+    public function lockPlatform()
+    {
+        $this->lock->set('php', [
+            'version' => $this->config->get('platform')['php'] ?? (PHP_MAJOR_VERSION.'.'.PHP_MINOR_VERSION),
+        ]);
+    }
+
     public function configureProject(Event $event)
     {
         if (!$this->downloader->isEnabled()) {
@@ -383,6 +390,8 @@ public function update(Event $event = null, $operations = [])
 
     public function install(Event $event = null)
     {
+        $this->updateAutoloadFile();
+
         $rootDir = $this->options->get('root-dir');
 
         if (!file_exists("$rootDir/.env") && !file_exists("$rootDir/.env.local") && file_exists("$rootDir/.env.dist") && false === strpos(file_get_contents("$rootDir/.env.dist"), '.env.local')) {
@@ -653,6 +662,36 @@ public function generateFlexId()
         $this->updateComposerLock();
     }
 
+    private function updateAutoloadFile()
+    {
+        if (!$platform = $this->lock->get('php')['version'] ?? null) {
+            return;
+        }
+
+        $autoloadFile = $this->config->get('vendor-dir').'/autoload.php';
+
+        $code = file_get_contents($autoloadFile);
+        $code = substr($code, \strlen("<?php\n"));
+
+        if (false !== strpos($code, 'PHP_VERSION_ID')) {
+            return;
+        }
+
+        $platform = preg_replace('/[^-+.~_\w]/', '', $platform);
+        $version = sprintf('%d%02d%02d', ...explode('.', $platform.'.0.0'));
+
+        file_put_contents($autoloadFile, <<<EOPHP
+<?php
+
+if (\PHP_VERSION_ID < $version) {
+    echo sprintf("Fatal Error: composer.lock was created for PHP version $platform or higher but the current PHP version is %d.%d.%d.\\n", PHP_MAJOR_VERSION, PHP_MINOR_VERSION, PHP_RELEASE_VERSION);
+    exit(1);
+}
+$code
+EOPHP
+        );
+    }
+
     private function fetchRecipes(): array
     {
         if (!$this->downloader->isEnabled()) {
@@ -839,7 +878,7 @@ public static function getSubscribedEvents(): array
 
         return [
             InstallerEvents::PRE_DEPENDENCIES_SOLVING => [['populateProvidersCacheDir', PHP_INT_MAX]],
-            InstallerEvents::POST_DEPENDENCIES_SOLVING => [['populateFilesCacheDir', PHP_INT_MAX]],
+            InstallerEvents::POST_DEPENDENCIES_SOLVING => [['populateFilesCacheDir', PHP_INT_MAX], ['lockPlatform']],
             PackageEvents::PRE_PACKAGE_INSTALL => [['populateFilesCacheDir', ~PHP_INT_MAX]],
             PackageEvents::PRE_PACKAGE_UPDATE => [['populateFilesCacheDir', ~PHP_INT_MAX]],
             PackageEvents::POST_PACKAGE_INSTALL => __CLASS__ === self::class ? [['record'], ['checkForUpdate']] : 'record',