Associative array corresponding to column name in a table - php

Question

<?php
header('Access-Control-Allow-Origin: *');
header('Content-Type: application/json');
include_once '../../config/Database.php';
include_once '../../models/Post.php';
$database = new Database();
$db = $database->connect();
$post = new Post($db);
$result = $post->read();
$num = $result->rowCount();
if ($num > 0) {
$posts_arr = array();
$posts_arr['data'] = array();
while($row = $result->fetch(PDO::FETCH_ASSOC)){
extract($row);
$post_item = array(
'id' => $id,
'title' => $title,
'body' => html_entity_decode($body),
'author' => $author,
'category_id' => $category_id,
'category_name' => $category_name
);
array_push($posts_arr['data'], $post_item);
echo json_encode($posts_arr);
}
} else {
echo json_encode(
array('message' => 'No Posts Found')
);
}
My problem :
I try to change $title to $the_title, it generates error, but when I change the column name in the table to 'the_title', it works!
My confusion is that since anything start with '$' sign is a variable, and it can be any name actually, but in this particular situation, why the name of the variable needs to be same as the name of the table's column.
$post_item = array(
'id' => $id,
'title' => $title,
'body' => html_entity_decode($body),
'author' => $author,
'category_id' => $category_id,
'category_name' => $category_name
);