如果从php调用，python webdriver将不会运行

我正在尝试从php执行python脚本。它在webdriver被称为[pytest.py]的行停止工作：

path_to_chromedriver = '/home/user/Desktop/2014/programs/chromedriver/chromedriver' # change path as needed
#i can print out something here (e.g. print 'hello') but not after the next line
br = webdriver.Chrome(executable_path = path_to_chromedriver)

这就是PHP调用python脚本[phptest.PHP]的方式：

$eventid_arg = $_REQUEST['eventid'];
$pyresponse_str = system("python pytest.py $eventid_arg", $retval);

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用户是否具有运行python webdriver所需的权限？权限很好。