Got it, let's break down what's happening and get your Composer working again:

Why This Error Happens #

The line causing the parse error uses PHP's null coalescing operator (??), which was only introduced in PHP 7.0. If your server/CLI is running a PHP version older than 7.0, this syntax will throw a parse error. Since Symfony Flex is a core Composer plugin, this breaks all Composer commands once it's triggered.

Symfony 4 itself requires PHP 7.1.3 or higher, so running an older PHP version is already outside the framework's supported environment.

Step-by-Step Fixes #

1. Verify Your PHP Version #

First, confirm your PHP version by running this command in your terminal:

php -v

If the output shows a version below 7.0, that's the root cause.

2. Temporary Fix to Restore Composer Functionality #

If you need to get Composer working immediately before upgrading PHP, manually modify the problematic line in Flex.php:

• Navigate to /Applications/MAMP/htdocs/myapp/vendor/symfony/flex/src/Flex.php
• Find line 275, which currently reads:

  $installContribs = $this->composer->getPackage()->getExtra()['symfony']['allow-contrib'] ?? false;
  

• Replace it with this PHP 5.x-compatible code:

  $extra = $this->composer->getPackage()->getExtra();
  $installContribs = isset($extra['symfony']['allow-contrib']) ? $extra['symfony']['allow-contrib'] : false;
  

After saving this change, you should be able to run Composer commands again.

3. Long-Term Solution: Upgrade PHP #

Since Symfony 4 requires PHP 7.1.3+, the proper fix is to upgrade your PHP version. For MAMP users:

• Open MAMP preferences
• Go to the "PHP" tab
• Select a PHP version that's 7.1.3 or higher
• Make sure your terminal is using the same PHP version (you may need to update your PATH or use MAMP's PHP binary directly, e.g., /Applications/MAMP/bin/php/php7.4.26/bin/php)

Once PHP is upgraded, you can safely re-run:

composer require security

to install the Symfony Security component without issues.

Note: Modifying files in the vendor directory is a temporary workaround—these changes will be overwritten the next time you run composer update or composer install. Upgrading PHP is the only sustainable fix.

内容的提问来源于stack exchange，提问作者Patrick Connelly