To get the first item of an array, you can use the reset function
http://php.net/manual/fr/function.reset.php
<?php
$posts = new Posts();
$listPost = $posts->feature_post($conn);
$firstPost = reset($listPost);
...
Also if you want to know if you loop through the first element and if the keys of your arrays are 0,1,2,3 etc..
<?php
foreach($array as $key => $cell) {
if ($key === 0) {
// this is your first element
....
}
}
If the keys of your array are not numeric indexes but you don't intend to use them, you can obtain such array by using the array_values function