composer install后出现如下错误，说php版本问题：
Problem 1

- symfony/browser-kit v3.4.20 requires php ^5.5.9|>=7.0.8 -> your PHP version (7.0.1) does not satisfy that requirement.
- symfony/browser-kit v3.4.20 requires php ^5.5.9|>=7.0.8 -> your PHP version (7.0.1) does not satisfy that requirement.
- Installation request for symfony/browser-kit v3.4.20 -> satisfiable by symfony/browser-kit[v3.4.20].