Update composer.json

Author: TeBoring

php/composer.json

@@ -6,10 +6,10 @@
  "homepage": "https://developers.google.com/protocol-buffers/",
  "license": "BSD-3-Clause",
  "require": {
- "php": ">=7.3.0"
+ "php": ">=7.0.0"
  },
  "require-dev": {
- "phpunit/phpunit": "^9"
+ "phpunit/phpunit": ">=6.0.0"
  },
  "autoload": {
  "psr-4": {