auto_calcPHP

<?php

$mysqli = mysqli_connect('localhost','xxxx','xxxxx','xxxx');

  $qry =  "SELECT * FROM calculation where title = 'fire_index' ";

$query = mysqli_query($mysqli,$qry)
  or die("Error: ".mysqli_error($mysqli));
//var_dump($query1);
    while($row = mysqli_fetch_array($query))
    {
     $title = $row['title'];
      $time = $row['time'];
      $formula = $row['formula'];
      $rtu= $row['rtu_name'];
      $sensor = $row['sensor'];
      $rtu1 = $row['rtu_name1'];
      $sensor1 = $row['sensor1'];
      $rtu2 = $row['rtu_name2'];
      $sensor2 = $row['sensor2'];
      $rtu3 = $row['rtu_name3'];
      $sensor3 = $row['sensor3'];
      $rtu4 = $row['rtu_name4'];
      $sensor4 = $row['sensor4'];



   $query1 = mysqli_query($mysqli,"SELECT $sensor as sensor from test where sn = '$rtu'");
        while($row = mysqli_fetch_array($query1))
    {
    $x = $row['sensor'];


   }

  $query2 = mysqli_query($mysqli,"SELECT $sensor1 as sensor1 from test where sn = '$rtu1'");
        while($row = mysqli_fetch_array($query2))
    {
    $y = $row['sensor1'];

   }

        $query3 = mysqli_query($mysqli,"SELECT $sensor2 as sensor2 from test where sn = '$rtu2'");
        while($row = mysqli_fetch_array($query3))
    {
    $z = $row['sensor2'];

   }


$step1 =  str_replace("x",$x,$formula);
$step2 =  str_replace("y",$y,$step1);
$step3 =  str_replace("z",$z,$step2);

eval('$output = ('. $step3 .');');



    }


?>