Hi Guys. I was browsing the Internet trying to solve my problem and finally I got to here. I have constants.php where I define my DB server, user, password and name, also I have connection.php where I parse these values.
constants.php
PHP Code:
// Database Constants
    
define("DB_SERVER""localhost");
    
define("DB_USER""root");
    
define("DB_PASS""root");
    
define("DB_NAME""widget_corp"); 
connection.php
PHP Code:
require("constants.php");
    
// 1. Create a database connection
    
$connection mysql_connect("DB_SERVER""DB_USER""DB_PASS");
    if (!
$connection) {
        die(
"Database connection failed: " mysql_error());
    }

    
// 2. Select a database to use
    
$db_select mysql_select_db("DB_NAME"$connection);
    if (!
$db_select) {
        die(
"Database connection failed: " mysql_error());
    } 
content.php
PHP Code:
<?php require_once("includes/connection.php"); ?>
<?php 
require_once("includes/functions.php"); ?>
<?php 
include("includes/header.php"); ?>
<section id="sidebar">
    <nav id="navigation">
        <ul class="subjects">
            <?php 
                
                $query 
"SELECT * 
                FROM subjects 
                ORDER BY position ASC"
;
        
$subject_set mysql_query($query$connection);
        
confirm_query($subject_set);
                
                while (
$subject mysql_fetch_array($subject_set)) {
                    echo 
"<li>{$subject["menu_name"]}</li>";

                    
$query "SELECT * 
                FROM pages 
                WHERE subject_id = 
{$subject["id"]} 
                ORDER BY position ASC"
;
        
$page_set mysql_query($query$connection);
        
confirm_query($page_set);

                    echo 
"<ul class=\"pages\">";
                    while (
$page mysql_fetch_array($page_set)) {
                        echo 
"<li>{$page["menu_name"]}</li>";
                    }
                    echo 
"</ul>";
                }

             
?>
         </ul>
    </nav>
</section>
<section id="page">
    <h2>Content Area</h2>
    
</section>
<?php require("includes/footer.php"); ?>
After I run my code I receive this error
Database connection failed: Unknown MySQL server host 'DB_SERVER' (2)

If I change all the variables in connection.php with their values it works fine. I use MAMP. Thank you.