Laravel 5.4发送邮件触发FatalErrorException错误求助

Hey there, I’ve dealt with similar compatibility kinks before, so let’s break down what’s going on and how to fix it.

What’s Causing the Error? #

The error message "带类类型提示的参数默认值只能为NULL" (translated: "Parameter default values with class type hints can only be NULL") stems directly from PHP 5.6’s limitations. PHP 5.6 doesn’t allow non-null default values for parameters that have a class/interface type hint—this feature was added in PHP 7.0.

The issue comes from the symfony/css-selector package: if it’s been updated to a version that requires PHP 7.0+, its code will use syntax that breaks in your PHP 5.6 environment. Even though Laravel 5.4 supports PHP 5.6, newer versions of its dependencies might not.

Step-by-Step Fix #

• Open your project’s composer.json file and look for the symfony/css-selector dependency. We need to lock it to a version compatible with PHP 5.6 (Symfony 3.x series works perfectly here).
• Update the dependency line to:

  "symfony/css-selector": "^3.4"
  

• Run this command in your project root to update just that package to a compatible version:

  composer update symfony/css-selector
  

• If you still run into issues, double-check other Symfony-related dependencies in composer.json—make sure none are set to versions requiring PHP 7.0+. Laravel 5.4 is built to work with Symfony 3.x packages.

Symfony 3.4 is a long-term support (LTS) version fully compatible with PHP 5.6, so it won’t use the newer syntax that’s triggering your fatal error when sending emails.

内容的提问来源于stack exchange，提问作者Ritchie