Composer PHP Version Requirements: PHP 8.0.0 Not Satisfying ^7.3

When working with Composer, you might encounter a confusing error message where PHP 8.0.0 doesn't satisfy a requirement for PHP ^7.3. This article explains why this happens and provides the correct solutions to resolve version compatibility issues.

The Problem Explained

The error occurs because Composer's version constraints use semantic versioning, and PHP 8.0 is not considered backwards-compatible with PHP 7.x in the context of dependency management.

The core misunderstanding stems from how Composer interprets version constraints:

• ^7.3 means "any version 7.3.0 or above, but less than 8.0.0"
• PHP 8.0.0 is a major version release with breaking changes
• Composer treats major version changes as potentially incompatible

This is why even though you have a newer PHP version (8.0.0), it doesn't satisfy the ^7.3 constraint that specifically excludes version 8.0.0 and above.

Primary Solution: Update composer.json

The correct solution is to update your composer.json file to explicitly include PHP 8.0 support:

{
    "require": {
        "php": "^7.3|^8.0",
        // other dependencies...
    }
}

After making this change, run:

composer update

This tells Composer that your project supports both PHP 7.3+ and PHP 8.0+, allowing it to install packages compatible with your PHP version.

TIP

Always check your PHP version first to confirm what you're running:

php --version

Why --ignore-platform-reqs Is Not Recommended

While this command may work temporarily:

composer install --ignore-platform-reqs

WARNING

Using --ignore-platform-reqs bypasses important compatibility checks and can lead to:

• Runtime errors from incompatible packages
• Unstable application behavior
• Security vulnerabilities from using untested combinations

This should only be used as a last resort when you're certain all dependencies are compatible with your PHP version.

Additional Considerations

Check Package Compatibility

Some packages in your dependency tree may not yet support PHP 8.0. Use Composer's why-not command to check:

composer why-not php 8.0.0

Update Lock File

If you've changed the PHP version constraint, you may need to:

composer update --lock

Verify All Dependencies

Check that all your packages support PHP 8.0 by reviewing their documentation or using:

composer check-platform-reqs

Best Practices

1. Always specify PHP version constraints accurately in your composer.json
2. Test with your target PHP version before deployment
3. Keep dependencies updated to ensure PHP 8.0 compatibility
4. Use version constraints flexibly when appropriate: ^7.3|^8.0 instead of fixed versions

By properly configuring your Composer requirements, you ensure that your application works correctly with your PHP version while maintaining dependency compatibility.