Get 2 variables in sql and subtract them in PHP - php

Question

I have multiple rows with data, some with data in mysql table kjoring and some that don't.
I am able to echo both $km1 and $km2 correctly, but I am unable to subtract them to find the result and echo $km0
$sqlq = "SELECT kjoring FROM oko WHERE kjoring!='' ORDER BY logdate DESC LIMIT 2";
if ( array_key_exists( 'field' , $out ) ) {
$sqlq = $sqlq . " where field = '" . $out['field'] . "'" ;
}
$conn = new mysqli($servername, $username, $password, $dbname);
$result = mysqli_query( $conn , $sqlq );
if ( $result->num_rows > 0 ) {
while($row = mysqli_fetch_array($result)) {
$km1 = $row[0];
$km2 = $row[1];
$km0 = $km1 - $km2 ;
}
}
echo $km0 ;

Answer 1

First things first: Little Bobby says your script is at risk for SQL Injection Attacks. Learn about prepared statements for MySQLi. Even escaping the string is not safe!
Next:
// $sqlq = 'SELECT ... FROM ... ORDER BY ... LIMIT'
if ( array_key_exists( 'field' , $out ) ) {
$sqlq = $sqlq . " where field = '" . $out['field'] . "'" ;
}
When you add your WHERE conditions after the LIMIT there WILL be a syntax error returned by your DB. You need to add the WHERE condition after the FROM and in front of the ORDER BY.
$sqlq = 'SELECT ... FROM ... ';
if ( array_key_exists( 'field' , $out ) ) {
$sqlq = $sqlq . " where field = '" . $out['field'] . "'" ;
}
$sqlq .= 'ORDER BY ... LIMIT ...';
Now to the 2 results:
while($row = mysqli_fetch_array($result)) {
$km1 = $row[0];
$km2 = $row[1];
When printing your row using var_dump() you will notice that there is only 1 column. You selected just one.
Using mysqli_fetch_all($result,MYSQLI_ASSOC); you will get an array containing both rows. Using this you can then do your computations.
PHP: mysqli_fetch_all()