您应该引用您的字符串，它可以正常工作，

You should quote your string, it works fine, see here.

$jsonString = '["m@gmail.com","b@gmail.com","c@gmail.com"]';
$arrayOfEmails=json_decode($jsonString);

或

$jsonString = "[\"a@gmail.com\",\"b@gmail.com\",\"c@gmail.com\"]";
$arrayOfEmails=json_decode($jsonString);