Update PHP in html form

Update: Given the short string of code you have provided, I shall make some minor updates to my answer.

Your question is very general, but I'll provide some code to get you started. For instance if you want to update the title for post #60.

HTML

<form action='update.php' method='POST'>
    <label for='title'>Post title:</label>
    <input type='text' id='title' name='title' required>

    <label for='content'>Post body:</label>
    <textarea cols='80' rows='10' id='content' name='content'></textarea>

    <input type='hidden' name='postID' value='60'>
    <button type='submit'>Update</button>
</form>

I'm assuming each post has a unique ID called id. Again this is a very simple example. For the sake of this example I'll also assume your database is MySQL. I will use PDO here but feel free to use mysqli, or the mysql driver if you really have to.

PHP (update.php)

<?php

    // Define your MySQL credentials here
    define('DB_NAME', 'your database name');
    define('DB_HOST', 'localhost'); // usually it's localhost
    define('DB_PORT', 3306) // usually it's 3306
    define('DB_USER' 'username');
    define('DB_PASSWORD', 'password');

    // Check if your HTTP POST parameters are set
    if(!isset($_POST['title']) || !isset($_POST['content']) || !isset($_POST['id'])) {
        die('Missing POST parameters');
    }

    // Create a new PDO instance
    $db = new PDO("mysql:dbname=" . DB_NAME . ";host=" . DB_HOST . ";port=" . DB_PORT, DB_USER, DB_PASSWORD);

    $sql = 'UPDATE `text` SET `title` = :title, `content` = :content WHERE `id` = :id;';

    // Create a new PDOStatement by preparing it
    $statement = $db->prepare($sql);

    // Bind the values to the prepared statement
    // By doing this you don't have to worry about escaping them
    $statement->bindValue(':title', $_POST['title'], PDO::PARAM_STR);
    $statement->bindValue(':content', $_POST['content'], PDO::PARAM_STR);
    $statement->bindValue(':id', $_POST['postID'], PDO::PARAM_INT);

    // Execute the prepared statement
    $statement->execute();

    // Since we're doing an update, it is a good idea to check if the query succeeded.
    // rowCount() should return 1 here
    if($statement->rowCount()) {
        echo 'Post title updated.';
    }
?>

For more information you can consult the MySQL documentation for the UPDATE query (or the documentation for your RDBMS).

Hopefully this will get you started!