如何从具有FOR循环的数组中提取特定行索引号？

I am not sure what I am doing wrong. The $numrows shows rows being counted, but nothing shows for the $Value1 $Value2 $Value3.

try {
$dbh = new PDO("mysql:dbname=database;host=localhost", username, password);
$dbh->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
} 

catch (PDOException $e) 
{
    echo 'Connection failed: ' . $e->getMessage();
}
$sql = "SELECT id 
        FROM table";

$result = $dbh->query($sql);
$numrows=$result->rowCount();
echo "Total ids found $numrows";

if ( $numrows > 0 )
{
   for ($i = 0; $i < count($result); $i++) {
            $Value1 = $result[$i]['id'];
            $Value2 = $result[$i + 1]['id'];
            $Value3 = $result[$i + 2]['id'];
            echo "Value1: $Value1 Value2: $Value2 Value3: $Value3";
    }

        Print '<br> test output';
}

You are iterating indexes multiple times :

on the first run :

$i = 0;

$Value1 = $result[0]['id'];
$Value2 = $result[1]['id'];
$Value3 = $result[2]['id'];

on the second run :

$i = 1;

$Value1 = $result[1]['id'];
$Value2 = $result[2]['id'];
$Value3 = $result[3]['id'];

etc.

You could do :

for ($i = 0; $i < count($result); $i+=3)

query will not work for this FOR loop. You have to use fetchAll. Using 'singe batteur's excellent counting code, we can complete this code:

$sth = $db->prepare($sql);
$sth->execute();
$result = $sth->fetchAll();
/* Fetch all of the remaining rows in the result set */

//print_r($result);  //check to see an actual array
//var_dump($result);

$numrows=count($result);
echo "Total ids found $numrows<br>";

if ( $numrows > 0 )
{
for ($i = 0; $i < count($result); $i+=3)
    {       $Value1 = $result[$i]['course_name'];
            $Value2 = $result[$i + 1]['course_name'];
            $Value3 = $result[$i + 2]['course_name'];
            echo "Value1: $Value1 Value2: $Value2 Value3: $Value3<br>";
    }

}