How to get the single value returned from a query and store it into a string

How to get the single value returned from a query and store it into a string

I have query that returns the employeeid from a query. How can I execute
the query and then store the single result in a string in php using sql
server. I am new and need the help.
Here is my query php code.
$queryFindEmployeeID = "select employeeid from employees where lastname =
'".$lastName."' AND firstname = '".$firstName."'";