可以读取cookie并在mysql SELECT Query中使用

$_SESSION["signed"]=true;
setcookie("cookie", $username);
$sql1="SELECT DR_id from `doctor` WHERE $_cookie['username']='$username'" ;

Table doctor

==========
DR_id|| DR_email      ||
01   || xyz@email.com ||

Can I get the id Through Cookie

Your WHERE clause should be a column name like WHERE DR_email='{$_COOKIE['username]}'

PHP keywords are case sensitive.

$_SESSION["signed"]=true;
setcookie("cookie", $username);
$sql1="SELECT DR_id from `doctor` WHERE DR_email ='$_COOKIE['cookie']'" ;