当使用PHPUnit测试的PHP代码中包含session_start()时，就会出现以下的错误提示：

PHPUnit 4.8.36 by Sebastian Bergmann and contributors.

E

Time: 110 ms, Memory: 12.00MB

There was 1 error:

1) ExampleTest::testBasicExample
ErrorException: session_start(): Cannot start session when headers already sent

解决办法是打开phpunit.xml文件，设置processIsolation属性为true