Proper way to generate random string in PHP and check whether it exists in an MySQL database

I am new to PHP, and I want to make a code that makes a new random string and checks to see if it exists in a database. This is my code so far. Is there a way to make it better or a more efficient way?:

function generateRandomKey()
{
  $randomList=array("A","H","I","P","Q","X","Y","f","g","n","o","v","w","3","4",")",
                  "B","G","J","O","R","W","Z","e","h","m","p","u","x","2","5","(",
                  "C","F","K","N","S","V","a","d","i","l","q","t","y","1","6","9",
                  "D","E","L","M","T","U","b","c","j","k","r","s","z","0","7","8",);
  $randomKey="";
  for ($i=0; $i<=8; $i=$i+1)
  {
    $rand=mt_rand(0,63);
    $randomKey=$randomKey.$randomList[$rand];
  }
  $sql="SELECT * FROM blogs WHERE blogRandomKey=".$randomKey.";";
  $result=mysqli_query($conn, $sql);
  $queryResults=mysqli_num_rows($result);
  if ($queryResults>0)
  {
    generateRandomKey();
  }
  else
  {
    return $randomKey;
  }
}

Any help is appreciated.