Query within a Query – mySQL & php

I am fairly new to php and mysql as I have been trying to teach myself. I am used to Classic ASP and am adjusting to php but I am struggling with doing a Query within a Query.

<?php
$con = mysqli_connect("localhost","username","password","database");
if (mysqli_connect_errno())
  {
  echo "Failed to connect to MySQL: " . mysqli_connect_error();
  }

$sel_query="Select * from accounts ORDER BY name asc;";
$result = mysqli_query($con,$sel_query);
while($row = mysqli_fetch_assoc($result))
{
    $refname = intval($row["referrer"]);

    $sql = "Select name from accounts where userid = '$refname' LIMIT 1";
    $username = mysqli_query($con,$sql)
    $row2 = mysql_fetch_assoc($username);

        echo $row2;
}

I am trying to perform a query of the entire table and then as I go though each row, I query the same table again to pull a name based on a referrer column.

How do I properly do this in php/mysql? Thank you