php - return the value of a database column from its ID

Solution:

Your not fetching the result, so it will be empty, jquery is also expecting json so you need to json_encode your response.

Also as your using PDO you should be using prepared queries.

<?php
$servername = "localhost";
$username = "root";
$password = "root";
$dbname = "test";

$json = [];

// is POST
if ($_SERVER['REQUEST_METHOD'] == 'POST') {

    // check user_id
    if (!isset($_POST["user_id"]) || !is_numeric($_POST["user_id"])) {
        exit(json_encode(['error' => 'Invalid user id']));
    }

    try {
        $conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
        $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
        $stmt = $conn->prepare("SELECT name_page FROM table WHERE id = ? LIMIT 1"); 
        $stmt->execute([$_POST["user_id"]]);

        $json = ['name_page' => $stmt->fetchColumn()];
    }
    catch(PDOException $e) {
        $json = ['error' => $e->getMessage()];
    }
    $conn = null;

} else {
    $json = ['error' => 'Sorry expecting POST'];
}

echo json_encode($json);
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For your jQuery try this:

$(document).on('click', '.update_btn', function(e){
    e.preventDefault();

    var user_id = $(this).attr("id");

    $.ajax({
      url:"fetch_single.php",
      type:"POST",
      dataType: "json",
      data:{
        user_id: user_id,
      },
      success: function(data) {
        $('#name_page').val(data.name_page);
        $('#user_id').val(user_id);            
      }
   });
});
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