SQL语句返回500错误

I have this SQL statement:

$sql = "SELECT CONCAT_WS (" ",Firstname, Surname) AS FullName FROM Applicant WHERE AppID=10010";

This statement returns Error 500 on my page despite of that it is a completely correct statement. I run this query on Sequel Pro and it is returning exactly what I need.

I tried a simple query and its working fine:

 $sql = "SELECT * FROM Cases";

I don't know why PHP is doing that. Maybe an extra pair of eyes can solve this.

wrong double quotes sequence

  $sql = "SELECT CONCAT_WS (' ',Firstname, Surname) AS FullName 
       FROM Applicant WHERE AppID=10010";

you must a different quote char inside the query string